[pesok]

can confirm with sprint, bought few from criagslist and ebay and never had problems getting insurance for any of them. Two things most people dont know/realise are:
1) You can get insurance at anytime within 30 days of any repair service, so you can bring your phone in for any problem(even made up) and since it "was serviced" you can get insurance for the device. Of course i dont recommend making up the problems to get the insurance.

2) most devices(like HTC TP/Diamond/TP2) carry 1 year warranty that starts from the time you first activate the device on the network(and not when you bought it/was manufactured). I bought TP a while ago from ebay and it was never activated. I activated it with Sprint 4 month later and waranty period started from the date it was activated. Just make sure its Sprint branded and was never activated(and has clean/clear ESN(even never activated/BNIB devices can have bad ESN if were reported lost/stolen).
Both points are only for Sprint and i am not sure they might work different on another network