Who knows about Sprint policies?

[testacon]

So here's my question...

If I buy a phone from ebay or craigslist and Sprint clears the ESN AT that moment in time and I buy the phone, if the contract that phone was purchased under goes on hold for non payment, how long after I add it to my account does Sprint still kill the phones ESN, rendering my phone useless?

Example, I buy a phone from Mr. Smith today, and Sprint clears the ESN and I add it to my account, 1/3/6 months later Mr. Smith cancels Sprints with a year left on his contract and doesn't pay his ETF, will Sprint dig up all known ESN's and kill them all, even know the phone is on another account now?