Not a bad plan. Great question. There are a few things to be cautious of.
1. Sprint tracks ESNs and will not activate an
ESN that has been stolen or reported lost. So ask your ebay seller if the the phone is kitted to Sprint and if the
ESN is clear.
2. Sprint's systems are set to automatically renew your contract upon
ESN change so you have to make it clear that you do not want to renew. They can override it but you'll need to ask and document who you spoke to, time and date, etc.
3. I would also make sure that your Sprint rebate eligibility would not change since you are not using it in this replacement scenario. That leaves your options open while you await the end of your contract.
Since you have until August you can check out the Samsung Instinct when it comes out. It's their answer to the iPhone and it's due out in mid to late June. The iPhone is cool but it was the first of its kind brought to the marketplace. Just go into it with open eyes. You can't replace the battery yourself (or even take it out from my understanding - correct me if I'm wrong forum), the network speed has been questioned, they've upped the memory capabilities because the market called for it, and I believe there have been some software issues and updates. Be sure to read the iphone threads.