The problem is the Open Market, then. Anyway, can anyone give me Chapter and Verse on where it says that magic items can be bought and sold, no problem? I only have Core 3.0, so I am probably out-of-date, but I would never allow the players to just pony-up money and get whatever they wanted. If they want a specific magic item, they have to either find it or (more likely) make it themselves.
Also, even if you can buy magic items, there's no guarantee you have any for sale of the exact type you want in the area you are. In fact, if the players' habits are any indication, the only readily-available items would be the "quirky" kind they're likely to sell rather than buy. Remember, the other guys are adventurers, too! Tracking down the rare magic item can be a quest in itself... the same as if they wanted to buy a castle or, say, a katana in Waterdeep.