As others have said, once you start getting into haggling it can screw with the assumed amount of equipment the players have. Man, if I started having interaction in magic item shopping, players would start thinking, "Hey, why don't I just steal it, and everything else in the shop? Bugger adventuring, looting shops is way more profitable!" and then I have to start statting shopkeepers. And if the shopkeepers can fight my PCs, that kinda lowers the value of the PCs - professional soldiers, mighty heroes, same power level as a shopkeeper.
I think I'll keep it abstract, lest I have to deal with these headaches. Magic item shops make little sense, but the game seems to need them and as long as everyone treats them as holes that you put gold in and get magic swords out of, it keeps the game balanced.