I find it a bit odd that people claim that Tressa is a terrible merchant, to be honest. She did pull off one of the most important tricks in the trade, after all: know the highest paying members of your "audience" and adjust your stock to match!
Is this from in-game or the players themselves? Nonetheless I think she’s skillful. She not only knows the true value of items but also prioritize the needs of her customers first. She could be a great sales associate in any retail store tbh.