I expect them to sell for more than they buy it for - that's to be expected. I am not intimately familiar with 5.0, because it is just poorly designed from what I am aware of, but the standard before that has always been that they pay half price on everything but trade goods. I expected half price - I sell for half the value but only pay the value. That has a factor of only 2 - something much more reasonable than 5.75.
That's exactly how it is defined in 5e. BG3 is following D&D in like 20%, most things you experience in the game is Larian's unbalanced homebrew.
