Online scammers have been using sites like Gumtree to cheat Canberrans out of cars, expensive jewellery and mobile phones - swindling up to $10,000 at a time, police say.

"A fair number" of investigations are underway, according to ACT police, following incidents where sellers had been fooled into giving away their goods, by buyers presenting false bank transfer certificates.

Police allege buyers had inquired about items for sale online and settled on a price with the seller, but when they came to purchase the items they showed the seller fake driver's licences and produced false online transfer certificates.

Tuggeranong station Sergeant Chris Meagher said often the buyer did not know they had been scammed until a few days later when the money had not appeared in their account.

"People have been scammed to the value of up to $10,000 in some cases," he said.

"These people are using online sites or apps, they'll arrange to meet a seller, be in touch by text message with the seller and it looks quite legitimate.

"But when they meet face-to-face a fake licence or ID is produced and then a text message is shown to the buyer on a smartphone to say the funds have been transferred into their account, which is in fact fraudulent."

Sergeant Meagher said police were investigating a couple of suspects and the incidents were not isolated to a specific area of Canberra.

"If you're selling larger items on Gumtree or any online auction site, check with your financial institution that the funds have been deposited first before you part with the goods.

"Also check that you're satisfied with the identity of the person who's meeting you and look after your own personal security."