OUTRIDERS

You want to do this alongside One Man's Junk...  and, more importantly, Inventor's Almanac . Selling items is easy enough, you just need to find any vendor, hit / to get to the sell tab and choose anything you don't need. You can select multiple items by hovering over each and hitting  to "mark" them, then holding  to sell everything marked in one go. You'll pick up well over 300 items as you play through the story, complete side quests and work on the expeditions for Hitting the Jackpot so there will be plenty of opportunities to get this. You can check your progress towards this trophy by going into the Accolade menu and going to Item Management. You need to sell well more than 300 items to unlock the highest tier of this accolade so this will at least give you a number visual of how many items you've sold/how many more you have to go. Once you've sold your 300th item, the trophy will unlock.

Important Note: Collecting Mods is far more important than dismantling or selling items so be sure to check which Mods you have or have not unlocked before selling anything. If it can get you a new Mod for Inventor's Almanac , do not sell it!