You’re right, the idea is to get loot to kill monsters. One of things I liked about D2 was that if I got loot I wasn’t going to use, I could go and try to trade it for gear that I would use. It’s still about the gear hunt, it just gives players another means to use gear they did find that is otherwise useless.
However, the key would be to have the drop rates set so the gear feels valuable, legendary items feel rare but not impossible to the point that trading feels mandatory to succeed. I never felt like trading was mandatory to succeed in Diablo 2 or Grim Dawn, unlike Path of Exile.
Trading can fit in an ARPG just fine if handled correctly. Seems to work just fine in Grim Dawn and Titan Quest, though the key difference is no leaderboards.
Remove the competitiveness of the game and perhaps those that are willing to use a credit card to get ahead won’t feel as much reason to do so.