Here's another scenario of the same type of scam:

Let's say you post an auction for a dirt bike, and one person offers to buy the bike very quickly.
To expedite the sale, the prospective buyer wants to have a third person (who owes him money)
send a certified check for more money than the cost of the bike directly to you.
The check will probably be for more than the price of the bike, the prospective buyer will ask
you to deposit the check in his or her account and then wire the back the excess money.
The buyer would then pick up the bike later. Here's the problem, the “certified” check is really
a high-class, hard-to-detect counterfeit.

Here are some warning signs that might help:

You are asked to act very quickly - Scammers don’t want you to have time to carefully verify whether the cashier’s check or certified check is authentic or counterfeit or to wait for the check to clear. Scammers typically ask you to wire cash as quickly as possible using an outside wire service such as Western Union. They know that their fakes are very professional and usually will pass an initial visual inspection at the financial institution taking the deposit. Some counterfeits, particularly those that involve identity theft of a real person, are so good that it may take weeks to identify the check as counterfeit. At that point, you are left holding the bag: the scam artists have your money and, under law, you are responsible for the missing money and may even be suspected of fraud.
Fake check scammers often claim to be in another country - That makes it difficult, they say, for them to send you a check directly and so they will have someone send a check on a U.S. bank. Often you are asked to wire the funds out of the country.
The deal is too good to be true - This old, smart consumer advice holds true in these cases. If a stranger is eager, sight unseen, to pay full price or more for the big-ticket goods you are selling and they’ll express a cashier’s check that afternoon, smell a rat. If after a few emails or phone conversations or even meetings, a stranger wants you to go into business, slow down.
Never accept a check for an amount greater than your selling price or fee - If the “buyer” sends a check in excess of your selling price, send it back and don’t send the merchandise.
If you accept a check even for the exact amount, “check out” the check - The name and address of the bank issuing the check will be on the check. If the bank is local, visit them and ask them to verify the check. If the bank is not local, locate the phone number independently (don’t use the number on the check) and call them to verify the check.

We at Aggieland Auctions wish to make your buying and selling an enjoyable and safe one.