Be sure you understand the percentages and the term "possible het" or "probable het". You'll see 50% probable hets, 66% probable hets, and 100% probable hets. A 66% probable het has a 2/3 chance of being a het, and a 1/3 chance that it is not. It either is or it isn't. A 100% probable het is advertised to be a het with NO chance that it isn't.
You MUST be careful who you buy hets from. Since they look normal, its easy to sell a normal as a het.
All that said, you mentioned 8ball. You can be very confident that you are getting what you pay for from Adam at 8ball. (edit) You beat me to it Jo!(end edit)
Steve