If someone deliberately misleads you to get what they want, they're a sandbagger. If you pretend not to know how to play pool so your opponent will bet money on a game, you're a sandbagger.
Sandbaggers deceive others about their real intentions or abilities, or otherwise use deception to gain something. In the law, if you know that a contract or warranty isn't valid, but you pretend to trust it so that you can sue later — you're a sandbagger. And in various sports and games, you're considered a sandbagger when you pretend to be less skilled than you really are so you can score higher or play with an unfairly high handicap.