Mahanak Market is one of the, if not the best, fruit markets in Bangkok, with a huge selection of fruit ranging from mango and pineapple to jackfruit and durian and everything inbetween - for super affordable prices! This is where locals go to buy fruit for themselves and shop keepers purchase huge bags of fruit through wholesale! Please see my travel tips section about pricing to see what we paid for the fruit above, as well as a general ballpark for what you should pay for various foods and how to avoid getting overcharged!
In general though Thai people are very honest (at least outside of the biggest tourist trap areas) and this market was no different. We found people extremely genuine, and they also all have prices displayed outside anyways so you'll find the sellers very nice and honest!