Scams involving checks (i.e. personal, cashier's, third-party checks, money order) take many forms. In a typical example, a thief posing as a car buyer “accidentally” sends a check made out for an amount higher than the selling price of the vehicle and requests that the seller deposit the check and return the difference via a wiring service (Money Gram, Western Union, etc.). After the seller has wired the money, he or she learns the buyer’s check is worthless, and the thief disappears with the seller’s money.
The Snopes page dedicated to check scams (including the famous strain originating from Nigeria) recommends waiting three weeks for any sizable check to fully clear. Even if it looks like the funds are available in the bank account before that time, checks could still turn out to be counterfeit.