Its not officially against the rules. Its been extremely frowned upon and many players have lost a lot out of shop exploitation. I'm not saying that its not wrong, nor saying that there shouldn't be a rule to protect shop owners, I just personally feel as if its the shop owners job to do their research on the servers economy around them so they don't receive such a big loss financially (I.E Price matching, Donation v Value ratio, cost efficiency+ price equilibrium)
Also want to add that there is a difference between shop price exploitation and scamming.
Also want to add that there is a difference between shop price exploitation and scamming.