Eric asks about it by the campfire and Rey refuses to answer. It's pretty clear that they stole something and want to barter it 2 weeks from then, but did I miss something or do we have no idea what it is and who they are selling it to ?
shareIt was while eating outside the motel that Eric talked about. He wasn't curious to know what the thieves did, I think he already had an idea. His goal was to point out to Rey that he was there, with Eric and his brother and mates weren't. Brief: they had abandonned him.
What kind of robbery they did isn't explained and isn't important for the story. It went out of hand and that led to the situation that Rey was left behind while the others went on a run, in panic.