Hmmm... without delving too much into my house rules or my reasoning, in my campaign essentially only potions are readily available , and then usually in limited quanities and only the cheaper sorts. Sometimes you can purchase items with 0th level spell effects (a wand of Frost Ray for instance, but more likely a Ewer of Chilling for making iced tea), and ordained clergy often have access to scrolls through thier churches.
Ok, a little bit of my reasoning. In D&D since the beginning, 1 sp has been the daily wage of an unskilled laborer. Several, but not all, assumptions were based on this. This resulted in a highly unrealistic economy in which labor was by far undervalued and many other unwelcome things occured. Now that XP is completely divorsed from collection of gold (under normal circumstances), we are free to fix this if we want to take the effort.
One question we might want to ask is how much a gold peice worth in terms we moderns can relate to. The answer is I think surprising. If 1 sp equates to a minimum daily wage, then a sp is somewhere around 40 dollars in the US. This means that 1 gp is worth about 400 dollars in 3 ed., or 800 dollars if you are using the more realistic 1:20 1st ed. exchange rate (a rate used through much of history). I like to round that to $1000 because it is easy to use. Therefore, a magic item, say a +1 sword, is worth around $2,000,000 in modern terms, say a million dollars if we use the official 1:10 exchange rate. For a second, we can leave aside what it really should be worth giving the effort that went into creating it. The point is, giving the obvious value of the sword, no one is going to invest in the production of one and then hope that a buyer comes along (before a thief does!). All high priced items like that will be custom ordered.
So if a PC wants a +1 sword, he doesn't simply walk into a store with a few hanging on the wall and pick out one. First, he has to find someone willing to take the time (and spend the xp) to make the sword. Essentially, he has to find a high level mage with a need for cash and/or sympathy for the character who doesn't already have work to do. This involves a Gather Information check with a modifier depending on the size of the community. Then he has to meet with the wizard and make the sale, usually involving a Diplomacy check. Then he has to wait for the sword to be finished. And this assumes that it is legal for him to a) purchase a sword in the first place and b) purchase a magic one. Many cultures in my world restrict the use of swords to titled nobility, and some restrict the possession of magic weapons (and many other kinds of magic items) to members of the government or governing heirarchy.