They don't cost enough new for me to waste the time w/ used ones. Now if you are getting them for like $20 and can put them in yourself to try them out, then i would say go for it. It kinda comes down to what your time is worth.
i think it depends on how the truck was used alot of weight in the back and driven hard 100k will tire them out, but i got mine from a soccer mom mall cruiser so @ 203K she still sits level.
but all i can really say is for me, i have 203K and still level and fine
Sitting level doesn't mean they have the control they once did and I bet taking a set of coils with 200k on them and comparing them to a new set there will be a world of difference and if the shocks have the same 200k then it is time for new parts.
My set of '99s was replaced at 110k and now with 45k on them they feel pretty firm compared to my mom's '99 Limited with 90k on the original coils plus her's are 11 years old compared to less than 5 years on mine.
i meant that they don't over compress when driving and are still firm, sorry i didn't elaborate and the shocks were replaced with bilsteins @ a150K or so.
Mine are good at 128K and brand new Trekmasters. Never have much in the back except the occasional move. Maybe .5 lower in back. Off-road once a year, but here it's more mud than rock--not that much flexing. Needed the new shocks badly, though.