OD&D
Among the supplements I know of with the best simple systems for use with PC Merchants are two from the Known World of 'Basic' D&D. I *think* they were available as PDFs from SVGames, at one point, but I do not know if they still are. Some shops may still have copies, or e-bay, 2nd-hand, etc.
The Minrothad Guilds, for Sea-based trading, and the Republic of Darokin for land-based (using the same principles).
On a more abstract level, the 2nd Edition Birthright campaign included Trade among the functions of Guild domains. The rules covered income for the ruler, but details for use in adventuring were left to individual DMs.
The 3rd Edition Arms & Equipment Guide from WotC also includes a very simple method. Basically, declare some places "over-supplied" and reduce prices, then declare other places "under-supplied" and raise prices. The rest is simply doing the math to see how much you can get buying-carting-selling.