If you are going to have unique, individual shops and merchants, then do up a list of about 10 items for each one. A pawnshop, a scribe, an alchemist. Other than that, if you're just keeping it generic, you can assume they can get any item within the gp limit as normal with a little effort. Obviously, depending on the climate or political situation or other factors some items might not be considered common or easy to find; heavy metal armor in the desert, unless the magic is to keep it cool, or necromantic items in a place that hates necromancers.
Otherwise, roll up about 10 items above the normal limit or that might be black market so you have something fast to reference in case a player does some shopping around or searching. Other than that, if they want something within the limit or more specific, they can have it made, but they'd need to wait for the crafter to make it (1 day per 1,000 gp in most cases) and likely need to pay at least half up front as a show of faith and to cover the creation costs. There's various ways to do it, but just having a quick list to reference should be fine otherwise.
This probably doesn't come up in most campaigns, but if you do roll up a really good (ie. expensive and out of their immediate means) magical item and the PCs find out about it... certain ones might go about sneaky or underhanded means to acquire it. So be wary on how freely your merchants talk about the stuff they have available unless they're already certain they're talking with customers who can afford their wares (typically that means customers who have bought their other wares and proven to be good customers). Don't just have them trot out the luck blade like they're showing off a goat or bolt of fabric.
