Where can I buy an used Iphone 4s for Sprint?

Where can I buy an used Iphone 4s for sale for Sprint? I'd check Amazon/Ebay but I'd hate spending $300-350 for a phone that could possibly have a bad ESN, or that'll have a bad ESN within a few weeks of buying it.

I own a GS3 under a contract for Sprint, so I'd rather not make a whole new line for an iPhone that i'll probably barely use as a main phone that much. My plan was to deactivate GS3 and activate the iPhone online on the sprint website.

Try Craigslist and meet the person in a Sprint store to do a switchover. I did this with another phone I bought off someone from Craigslist and once the store did the switchover, I paid the guy the money. There are 2 advantages to this:

1. You are guaranteed to get a clean ESN phone since Sprint will not switch it to your account if it does not have an ESN.

2. You are doing the transaction in a public area with witnesses. If he tries to rob or scam you, you have the workers and maybe the store's cameras as well to witness it.