The warning comes following a daring raid on a garage in Manorcunningham at the weekend in which three expensive cars were stolen.
Gardai have now released more details of the raid in which €93,000 worth of cars and jeeps were stolen.
They revealed that five raiders took part in the robbery and all were wearing balaclavas to avoid CCTV footage.
The raiders spent two hours at Colm Dillon Cars in Manorcunningham in which they took two Audis and a Toyota Land Cruiser jeep.
Gardai say the vehicles, a red Audi A4 worth €27,000, a 2008 red Audi A5 worth €26,000 and a grey Toyota Land cruiser worth €40,000, were low in fuel and have appealed for any garage owners who may have been used to fill up to come forward.
Garda Supt Vincent O’Brien said they believe raid took place around 1am.
He said the Gardaí are working “very closely” with the PSNI and have “made significant progress” in their investigation.
Supt Garda O’Brien added “It would appear that garages are being targeted to steal cars to order. This is the second raid of this type on Dillon’s. In May, a 2009 Toyota Corolla was stolen and this vehicle was later recovered, on June 23, in Ballyshannon. The car had been prepared for sale and had false number plates fitted.
“We would advise anyone intending to purchase a car off a website or from a magazine to be wary, especially if the seller gives a mobile number instead of a landline for a business or wants to meet in a car park. Check the chassis number and always get Registration Book or Certificate.”
Anyone with information is asked to contact Gardaí in Letterkenny on 074 9167100.