This trophy will require grinding. Each sale must be at a different market (station or planetary base), but they can be in the same system. My recommendation is to use the navigation computer to select a system you can jump to, then look at it on the galaxy map. If you change the filter to commodities, the game will tell you if you've ever docked at a station there before. If you have, you'll be able to see the station names in system even if they don't trade the good you have selected for the filter. If not, and it is inhabited, go there, dock at every station, and buy and sell back 1 item at each. Then check the computer again. To make it quicker, I recommend using a Small ship (as the smaller stations only have one Medium pad, and sometimes an NPC is sitting on it for a minute or two after you arrive) and avoiding planetary bases and stations that are over 5000 Ls from the jump in point. It's faster to jump to another system and find a close station than to fly to a station at the third star of a trinary system that's over 600,000 Ls away! Also, when targeting the station in the navigation computer, make sure it says "Trade" at the bottom, as some stations do not offer commodity trading. You can check your codex for trading stats, and it will give you a count for this purpose, but be aware that it lags, so if the trophy doesn't pop right after your 250th different market sale, fly to another station or system, or log off and back on. Good luck, and o7 CMDRs!