They can but run the risk of not getting full value due to condition, dirty/under contract, etc. and if Samsung rejects it you can be charged a fee.
super small risk imo. Most big sellers tell you condition etc if you happen to get one that comes bad return it before trying to send it to samsung.
you can still buy brand new unlocked for $450. Im not saying you cant sell yours and i was thinking of selling mine as well but i just don't see the value here