Buying a Used iphone battery question

Looking to purchase a used iphone on ebay mainly b/c i want a 4s 16gb and they dont have this available at virgin mobile online and thats the carrier i want.
Assuming the battery isn't that good... i could always go and replace it and have a brand new battery right?


How much does the new battery cost and to have it installed?

okay so if you buy that... that mean the battery would be just like it was brand new correct?

Im curious but for those of you with iphone... assuming you keep it for long time... how long often do you need to replace battery?