The riskiest of risks.

If it's a reputable retailer, like TNT Amusements for example, you can rest easy that you'll receive a machine that's been gone through and warrantied, and you'll certainly receive the machine you paid for.

Private parties can be tricky. Unless you're going in person to inspect a game, with cash in hand, and the ability to haul it out after the cash exchange, it's usually not a good idea to make purchases sight unseen.