Picture special: photography by Nick Ponty

Everything in this castle is for sale: the paintings, furniture, silverware and ornaments; the swords and sabres by the door; the letter from Mary, Queen of Scots; the portrait of the severe-looking gentleman who inspired the phrase "Bob's your uncle"; even the bizarre-looking chair that was – although you'd never guess it – an early form of exercise machine.

In the next few days, all of this will be taken out of the house – Blair Castle, in Ayrshire – auctioned off in Edinburgh and then split up for the first time in centuries. It's a 900-year-old story, and it's all for sale.

Standing by the front door to the castle, owner Caroline Borwick says she agonised over whether selling was the right thing to do but in the end knew it was. "I'm peaceful with it," she says. She then looks sheepish but decides to say what she was going to say anyway: "The house is peaceful with it too, because I've asked it. I'm not superstitious but I do think old buildings talk to you."

Caroline Borwick at Blair Castle

Today, Borwick is showing me around the building, near Dalry, accompanied by Gavin Strang of Lyon & Turnbull, the auctioneer who has spent the last few months valuing the contents (worth at least £500,000), and the exuberant Borwick dogs who scamper in between the furniture. For the last six years, Borwick and her husband Luke have been running this house as a home and business, renting it out for weddings, corporate events and luxury getaways.

The idea for the business evolved after the couple inherited the house from Luke's aunt 11 years ago. Borwick says the house had been slowly dying before that and they promised the aunt they would revive it: the business they built was their way of fulfilling that promise.

So why sell? Blair has been lived in by the family for almost 1000 years: the family tree was planted here, flourished here, and even though it withered during the First World War, it survived right up to the Borwicks' residence. That long, uninterrupted line in one single place is extremely rare in Scotland. In fact, this is the oldest continually inhabited mansion in Scotland, so it can't have been easy for the Borwicks to decide to sell. The lady of the house says she'll tell me why, but first wants to show me some of the treasures.

Treasures with colourful stories

We start just inside the front door where there's a trunk on the floor. Borwick opens it to reveal a pile of swords inside. She picks one up: it turns out to be the sword of a pirate-slayer. She passes it to me and there's a undeniable buzz: a thrill at holding history. The 19th-century weapon, with its beautiful lion's head and ivory grip, belonged to Captain William Fordyce Blair, the 25th laird. Part of the problem with running a house like this is how to pay for it and the captain's solution was to capture pirate treasure. There's a portrait of him upstairs, buttons shining and sideburns bristling.

Borwick picks up one of the other swords and this one is even older: it belonged to Major Hamilton Blair, the 23rd laird, and there is another colourful story associated with it. It is said Major Blair fell into a row with the locals and was forced to hide in a ditch to escape the mob, where he caught the cold and died. Borwick didn't know that story until, while sorting through the contents of the house ahead of the sale, she found a letter from the major's great grand-daughter talking about it.

I ask Strang who is likely to buy these swords. "A combination of people," he says. "There are collectors but, on top of this, we can connect with interesting people. Sometimes these swords appear and you don't know their history, but here we do."

It is also likely that some of the contents will sell to the 35,000 families around the world who can trace their DNA back to this place: Blairs who want a piece of Blair. And there is everything, from the relatively cheap – a bust of Napoleon, a Punch-shaped doorstop, even the garden watering cans for £50-100, right up to the impressive and expensive.

A selection of Blair Castle treasures

One of the best examples of the latter is a grand early-19th-century painting of a day at the races at Leith by James Howe, which is likely to sell for £10,000-15,000. It's an impressive landscape showing the crowds cheering on the racers. What's unusual is that it isn't a stiff, formal portrait like many of the other paintings in Blair – this is a social document of life in Scotland. It's full of mud, blood and grit. It's real.

The painting hangs in the dining room which is where the Borwicks and about 100 guests had their last party at Blair a few weeks ago: their farewell to the house. The house itself, and some of the land, has been sold to the English Conservative MP and Energy Minister Charles Hendry and his wife Sallie for something under the £2.5m asking price and Borwick says the new owners intend to run it along the same lines, which is a comfort to her.

Which brings up that question again of why the Borwicks decided to sell. Is it because it hasn't worked as a business?

"Not entirely," says Borwick. She says the business has gone well but renovating the house was a huge, five-year task. She and her husband had to run it with minimal help and there are glimpses of how tricky that has been when Borwick complains about some of the damage done by guests. But she's practical, she says, and realised that at 62 (her husband is 64), they would have to sell before they got too old to run it.

"You have to be realistic about these houses," she says. "This is a beautiful house, but we need an alternative income and the last two generations didn't. So the question for us really was: having revitalised the house, what's the future? And, to us, it's important the house goes on living. It doesn't go on living with people getting too tired and too old. And we didn't want to end up like that. It had happened for two generations and we didn't want it to happen to us."

The other issue is that the Borwicks' children didn't think they could take over. Their daughter Alexandra is a Christian missionary in Texas, while their son Malcolm lives for much of the time in Argentina, where he is a polo player and friend of Princes Harry and William (there are photographs of them all together on one of the sideboards).

The contents of Blair will be on display from February 24

"I think it was about finding someone who would love and care for the house," says Borwick, "and our children didn't see how they could do it."

Besides, in a way the Borwicks aren't really leaving this place as they have converted the nearby carriagehouse into a home. They have also made a film that captures the house in its present state, just before the auction. In total, the sale is expected to raise about half a million pounds and one of the items likely to attract most interest is the portrait on the stairs of an old major with an interesting story, a remnant of which still lingers today.

We go and have a look at the portrait and he's a fine-looking fellow, Field Marshal Frederick Sleigh Roberts, but it's his nickname that provides the clue to his story: Bobs. It's said that old Bobs so cared for the welfare of his men during the Battle of Kandahar – the final battle of the Second Afghan war in 1880 – and other campaigns that he gave rise among his soldiers to the phrase "Bob's your uncle". It's a nice touch that: a touch of modern among the old.

We head back downstairs and there's still time for Borwick to show me that strange, early exercise chair. It has handles on either side and a cushion in the middle that goes up and down. The men of the house would apparently use it to prevent the onset of gout. It is, essentially, a Regency version of the cross-trainer and is expected to fetch up to £1200.

Its buyer, and those who will buy the rest of the contents of Blair, is still uncertain but in a few days the doors will be open to anyone interested in having a look. The sale will take place next month and it's obvious Borwick has come to terms with that. She says they could turned the house into a museum but it's in the wrong location for that and anyway it's time for change.

It's also the end of today's visit so we head outside and stand around in the courtyard for a few minutes chatting. "I don't think it's ever productive in a business sense just to keep churning away," says Borwick. "You have to be going somewhere."

Does she know what her next project will be? "Good question," she says. "I don't." And then she stands at the front door to wave me off, one of her dogs at her feet, like a statue.

The contents of Blair will be on view to the public from February 24-26 (see website for details). The auction will be held at Lyon & Turnbull, 33 Broughton Place, Edinburgh, on March 14-15. Visit www.lyonandturnbull.com.