The list of retailers that wont rip you off would be too short, and the list that would rip you off (black list) would be too long.
You have to gain the Thai retailers trust and friendship before they will treat you as an equal and give you fair prices. I have been here 2 1/2 years and still run into a select few that try to pull a fast one. I just walk off to let them know that I am on to them.