We've bought from Amazon before. Are you buying this with a Sprint plan?
If you're buying it with a plan, Sprint should be able to service the phone for you if something happens.
I believe they send you the phone at that price, and you have to activate it with Sprint within a certain amount of time (30 days?), or your card gets billed for the full device price (whatever the penalty is, should say there). After that, it's a Sprint phone. Anything happens to it, you can take it to any Sprint shop and they should take care of it.
We ordered from Amazon a few years back and it was a VZW plan, but I doubt it's much different. Everything should be there on the fine print.