The cheap imported devices could also pose a significant risk to dentists and dental nurses, health officials said.
The Tianjie Dental Falcon scanners can be bought on eBay from £205, compared with around £4000 for a safe model.
Officials said one UK-based dental practice bought one of the machines, and several others had expressed interest.
Any dental practices that have bought the machines have been urged to contact the Medicines and Healthcare products Regulatory Agency (MHRA).
Bruce Petrie, of the MHRA's medical devices enforcement team, said: "We have seized 13 of these X-ray machines from the distributor and we are working with eBay and other governments to ensure dentists and patients are protected."