My own take on this is as follows:
Only cheap and commonly used potions would be available in a trading post. A potion of healing is relatively cheap and is usable by everyone. There's enough of a market and a low enough cost that you can buy one and count on selling it to someone else very soon.
About the only other items that would fit that would be some low level scrolls (it would need to be a big merchant counting on going to a large market because most people can't use scrolls) and wands of cure light wounds (again, it has to be a big merchant going to a large market but such items are in pretty high demand).
For everything else, you need to find it, make it, commission it, or find someone else who is selling it.
The last is what will often happen. Another group of adventurers goes to the mountains of infinite kobolds and gets a cloak of resistance +1. Since they want to split the treasure evenly and that's the only item they got, they decide to sell it and split the proceeds. So, they pay for a reputable wizard to identify it, Arcane Mark it, and sign a notarized certificate of identification. Then they do some legwork, talking to people who might be interested in such a thing and generally publicizing the fact that they have it and want to sell it. They might contact a broker of such items to use his contacts. Then they meet the buyers, negotiate a price and sell the item.
Of course, the list of people who might sell you item X also includes people who don't have item X but know where they could get one. So, a rogue might end up in hot water if he tells his guildmates "I'll give 4000gp for a +2 cloak of resistance." He's likely to get the cloak but the previous owner might come looking for it.
In practice, however, you don't want to regularly have adventures about selling magic items, so you'll just assume that it works properly until you want to run such an adventure.