Make sure and look him/her directly in the eyes,and lead off with a firm handshake. Never agree to the first offer price. Always haggle. You would be surprised. They expect you to,and if you don't they will think you are weak and will rip you off. Don't even get me started about cricket dealers!
Decide what species you are going to get. Research husbandry, and get his / her new home all set up before purchasing the cham. Seems like a lot of people get the cham first, then can not house it properly.