The general rule is to look at the price of a reasonable substitute. The fact that the item is used shouldn't make a difference. If you breached a contract to sell me your 2003 BMW with ~50k miles, I can show its market value by finding other 2003 BMWs with ~50k miles for sale.I agree with BoredAtty in detail, but point out that it is difficult to find a market price for a used item. To calculate the fmv of a used item, zinger's point about how much the vehicle sold for is valid. An arms-length sale soon after the breach is a good proof of value.
If there are no reasonable substitutes such that it is difficult to determine the market value of an item, then I agree that the sale of the actual item is one way to attempt to prove market value. However, even when no substitutes exist, a recent sale is certainly not conclusive proof of market value.