The Michelle Thomson affair may not tell us much about the state of the Scottish National Party, but it has shone a revealing light on the property business. It isn't for the faint hearted.

Now, first off: buying properties at a low price and then selling them at a higher price is just what property companies do. It is called arbitrage. Indeed, buying low and selling high is what most businesses do.

Ms Thomson has not been accused of any illegal or fraudulent practices. However, her property lawyer, Christopher Hales, was struck off by the Law Society of Scotland for being engaged in 13 property transactions he did not report as potentially fraudulent. Ms Thomson was involved in all of them.

Homes were bought from distressed sellers, one of them a cancer sufferer, and sold on within days. Properties were remortgaged at a higher valuation, in a practice called "back-to-back" selling,

Here's how it works: I buy your house for £250,000 and I sell it on the same day to an associate for £315,000. This person then takes out a mortgage at that new valuation, and hands several thousand pounds back to me; nice little earner.

Such “cash-back” deals – when buyers agree to give money back to sellers or vice versa – are not necessarily illegal. But they can be a sign of fraudulent attempts to secure inflated mortgage loans.

Back-to-back selling was used by criminals to launder money during the property boom. Money magically appears as if from nowhere as a result simply of the rapid sale of assets. Indeed, in cities such as Manchester, drug money is thought to have helped fuel property inflation in the city

No one is suggesting that Ms Thomson was laundering money or acting criminally. But she was involved in deals that, under the rules of the Law Society of Scotland, should have had alarm bells ringing.

When loans are sought from mortgage lenders for higher sums than the immediate purchase price of the home, or where the property is recorded on the Scottish land registry at a higher value than was paid for it, lawyers are under an obligation to report these practices.

At the very least, mortgage lenders such as Birmingham Midshires, should have been made aware that the properties had been purchased for a much reduced price less than six months previously.

It may still be the case that Ms Thomson was unaware of the practices, was wrongly advised or was set up. Simply buying property cheap and selling it on for a profit is not illegal in itself, even if you do it in a matter of days. If it were, the stock market itself would be illegal.

There may have been other factors involved that we don't know about. Indeed, Ms Thomson appears to believe she was actually helping distressed people who were unable to sell their homes, though some of these sellers are telling newspapers that they felt cheated. She needs to explain how her property speculation was actually benefitting those sellers.

The SNP's former business spokeswoman has, however, resigned the SNP party whip and will sit as an independent until her name is cleared. This will not be easy. She hasn't spoken about the deals or tried to justify them.

And the real question is whether she can ever be free of the taint of exploitation that hangs over the whole property business like a dark cloud.

Now, the SNP is not a Marxist party. Indeed, its former leader, Alex Salmond, is a firm advocate of responsible capitalism and a former employee of the Royal Bank of Scotland.

During the crash, RBS bought up property and businesses at distressed prices and then sold on when the market recovered. Banks like RBS were heavily involved in sub-prime property dealing.

This isn't illegal. And at the Wigtown Book Festival, Mr Salmond was adamant that Michelle Thomson should be reinstated as soon as her absence of formal guilt was established.

Others aren't so sure. Some believe she is "damaged goods". Property speculation is tainted. Those involved in it do not make anything; they do not add any value; they do deals.

Where will this end? Many SNP MPs have property interests, including buy-to-let. It is sometimes claimed that renters are "exploited" by buy-to-let landlords who have tax incentives to purchase all the available housing stock.

It would be ludicrous for every MP with a second home to resign. Nevertheless, their property holdings will all now have to be examined.

I do not envy the party official who is charged with investigating his or her own MPs. The law isn't much help. In politics, sleaze is often in the eye of the beholder.