A classic scam has been making the rounds on websites such as Craigslist and Kijiji -- someone contacts you to purchase an item you’re selling, and is even willing to send a cheque for more than it’s worth. In return, they only ask that you use the extra funds to ship the item via a third-party courier.

It may sound great, but as is so often the case, it’s too good to be true. Here’s how the scam works:

When a trusted customer deposits a cheque to their bank, most institutions will immediately credit the funds to the account. As a result it looks like the cheque has cleared, the funds are there, and the seller can feel confident about concluding their end of the deal. So they send the item, along with the balance of extra cash provided to cover the shipping costs.

The problem is that it takes about a week for the fraudulent cheque to bounce, but by then it’s too late. The item has been sent, along with the cash which ends up coming out of the seller’s own pocket.

They may also get hit with any charges from their bank for trying to cash the fraudulent cheque.

Thomas Keenan, a computer sciences professor at the University of Calgary, said many people have fallen for the scam over the decades since it first emerged as a phone and mail scam.

"People are gullible, I guess it's that simple. We like to trust other people. When you use Kijiji, Craigslist or any of those sites, remember it's a person on the other end, it's not a robot and that could be a good person or a bad person, and these folks often have a really good yarn," Keenan told CTV's Canada AM.

Here are Keenan's tips for avoiding falling victim to this scam or others like it:

Meet face to face: If you're going to sell or buy an item online, try to arrange an in-person meeting to view the item and make the exchange: '"I once bought a computer in a doughnut shop. The guy said let's meet there and that's fine as long as you're able to actually verify the goods," Keenan said.

Avoid sales or purchases that involve Western Union: While the money-order service is invaluable in a pinch when sending money overseas, it is often targeted by fraudsters because transactions are untraceable and can't be cancelled. And once money is sent, the recipient only needs to know the answer to the secret question in order to collect it.

Be careful of PayPal receipts: Scammers have found ways to forge fake PayPal receipts that look like the real deal. They then email the receipt to your inbox from a fake account, and it appears as though the money has arrived. Once the item is shipped, it's too late to get it back.

Use common sense: If you can't see and touch the merchandise, you're likely dealing with a scammer, Keenan says. If the seller tells you they're out of town and will ship the item, or if a potential buyer offers to send a fat payment to cover shipping costs, beware, you are likely speaking with a scammer.

Avoid any ads that ask you to send your address: "There are actually fake ads on Craigslist that are only intended to get you to give out your address because your address is worth money," Keenan says.

The bottom line, he said, is to remember there are many fraudsters out there trying to capitalize on online commerce, and often the laws and police investigative tactics haven't kept pace with scammers' skills.

Police are often at a loss as to where to even start their investigations, he said.

"The weirdest one I ever heard of was a guy who went on Craigslist and he marked a job advertisement as spam so no one else could apply for that job, he went out and got the job. There are weird people out there."