I do /shop a lot to sell stuff and buy stuff. I have noticed that buying stuff costs about FIVE TIMES what it costs to sell stuff! I don't think this is at all fair. In real life stores are all about having a good customer experience and fair prices. I think that the /shop should be like that. If someone accidentaly buys something, they cant return it for their money back.
an example is iron, to buy one ingot it is 100$ but to sell one is only 20$. Also bookshelves, they are 135$ to buy but only 15 to sell