I sometimes cannot buy an item, like, for example:
Iron Armor of Power. Cost: 26040 - I had more than ten times as much money in my inventory Level: 22 - I was level 22 Need str: 12 - I had 16 Str unmodified.
Was there a buy button next to it? If there is one then your browser session was probably messed up and you need to refresh (if you are using IE, add "www.playmage.com" to trusted site if it happens often). If there isn't a buy button, then it's likely a but that is not updating, and you should just need to exit the shop and go back in to fix it.