I don't forbid the buying of items, but I don't include magic marts. You'd be surprised how easily taking the magic shops out of the game takes away the issue.
I want to buy a magic item.
Ok, what kind?
A magic sword.
OK, just any magic sword?
I dunno, a good one.
What's a "good" magic sword?
I don't know, a +1 sword.
Ok cool, you want to buy a +1 sword. I'm sure someone has one that would rather have money. Where are you going to start?
What do you mean?
Where are you going to start looking for a seller, or are you going to put out an ad in the Gazette?
At this point we either get to go on an adventure to find a magic sword owner who needs money, or they just give up. So far they've given up every time, because 99% of D&D players would rather search a dungeon for a magic sword than play the exciting game of the Fantasy equivalent of the high art trade.