The closest thing to a market place I remember caring about as a kid was the used video game exchange program at a store equivalent to Gamestop. I remember realizing how rigged it was because I had to bring in 3 or 4 used games to get just one in exchange...
So maybe some kind of trade like that but using Craigslist, Facebook or Ebay to trade with other kids?
For example, they could buy a big bundle (someone getting rid of everything at once : their gaming system + all their games + the controllers, whatever) then resell each element separately. It takes work and patience to collapse the "holding discount" (deferred gratification) and if let's says they manage to recover their initial investment and still have a couple games left afterwards, essentially getting them for "free", I bet that would feel pretty good to an 8 years old.