Bikes are excellent value used. You have to know a bit about it to make a good decision, but bikes tend to be super sturdy, and their value goes down after a few years. Bikes are definitely my favorite piece of "gear" to buy used.
Otherwise, I think REI's used section typically has only stuff in good condition. I wouldn't buy a helmet used, but stuff that's meant to last, like packs, tents, some clothes, etc. should be safe