with iphones avoid 3g and 2g cause 99% of 3g and 2g in the market have been repaired in someway or the other . .. battery life of an iphone is max 1.5yrs after that the battery backup is really less ..
with 3gs also i would never buy ones more than 1-1.5 yr old..
if ur getting a fac unlocked 3gs below 20k then def somethin is wrong there ..and chnces of it having problems are very high .u can be lucky and get one really cheap and working well but u have to be really lucky for that .
when u buy a iphone make sure u buy one that is not more than 6-8 months old . repaired iphone completely avoid cause even apple doesnt reapir the iphones they just replace it.. reparing ruins the iphone .
its not about the price when u buy a used phone ,its also about the phone working well .. dont hesitate to pay a few hundred more if the phone is in good condition cause at least it will work well for a few months till u resell it to some bakra