if he is selling to get something stiffer i can only assume he rode them really low and wants something stiffer to go even lower. this means that in the 16k miles they were on the car they got ****ed up.
rebuilding a set of coilovers costs in the area of at least a few hundred dollars including you shipping them to the manufacturer. at that rate you might as well spend another couple hundred and get a brand new set. dont get me wrong, if he rode his car around on a 2" drop and just drove the car (no tracking of it) then they are probably fine. also, consider what car they were on and how often they saw the street. if they were on a DD all of the locking rings are probably frozen solid and you won't be able to break them free unless you break them off.
either way, its up to you. i just think it'd be more of a hassle than just buying a new set for $1100.