In my last campaign, there were basically two guys who sold magic items in a roughly 100 square mile area. My PCs (of course), found both of them.
The first was a traveling wizard with an Orc bodyguard.
The wizard had a specific inventory (which I worked up beforehand) in a locked, magically locked chest which he only opened for a fee. In other words, you couldn't browse his stuff unless he knew you were serious about making an acquisitiion.
Secondly, the orc had some spellcasting ability (I never statted him out), but he had a wand of zone of truth. When the PCs wished to make a bargain, the orc would cast zone of truth. The wizard would ask "have the items been identified?" and "Is the item cursed?" and "What does the item identify as?". He would ask whether the item had been identified prior to starting the transaction. It would have been a waste to cast the ZoT only to find out the PCs hadn't yet identified the items. He could, of course identify them for a fee if the PCs needed that done.
Once the ZoT was cast, the PCs could reveal what the items were to the wizard, he would unlock his chest, and the trading and haggling began. He had set prices for all of his items (textbook costs, basically), and would offer 50% market for items purchased.
The second such individual was an elderly elf antique dealer the PCs found in an Elven sea village. Walking through his cluttered store, the layer of dust over all the antiques led one to believe that the antiques were a mere display for other kinds of activities that the merchant dealt with. In the back, the elf (a fairly high level wizard, now retired), had a number of items. He didn't carry large amounts of coin, so he was more interested in trade than in outright purchase of items. Again, I had a set inventory, and costs.
In both cases, I included "Wow" items that the PCs would have wanted badly, but would have been out of their price range. I included items they might really need down the road, and I would usually include a few generic weapons and armor type things, and then usually one really weird sort of thing that I might not hand out normally, or might not show up in a standard loot pile somewhere. Just for fun.