Caveat Emptor.
It's possible you can purchase an iPhone from a pawn shop and everything will go fine activating it with your provider... it's also possible you could wind up with a device that can not be activated because of one of any number of issues... if you're interested in a Verizon or Sprint model, try to get the pawn shop operator to verify the ESN is clean... or guarantee a return & refund if you can't activate it on your account. If it's an AT&T model, get assurance the sim slot is unlocked and get a guarantee for return & refund if there are issues with getting it activated on your account.