IMC, players can and do purchase any magical item. However, it isn't as easy as it seems. You've got to find someone who has the item and is willing to sell it. Some campaigns have actual magic shops, and some don't. Even when there are magic shops, these will usually only reliably stock relatively common items. For stuff such as powerful items, or items that aren't especially powerful but are still rare (wands and potions of seldom-used spells, weird wonderful items...), the characters have to search, sometimes pretty hard.
Basically, I play it according to market logic, or at least try to. I think that it is pretty silly that characters could find any magic item in a ready fashion, and equally silly that (if the campaign is high-magic enough) there is no shop specializing in magic items.