I work in the business and can tell you a bit about this. An unlocked phone is not altogether a bad thing, but there can be some drawbacks that make it a bit of a crapshoot. If you toss out ebay seller issues, which may not be insignificant in terms of getting what is really described (new versus refurb, etc.), the main thing you're looking at is the phone you buy will likely not have your providers latest software version, this may mean some quirks or issues that can range from small (you get a text message of some bizzare symbol every time you check voicemail) to large (you don't have all the service functionality offered by your provider)
I have had friends buy phones off ebay and have no issues, moderates issues, and so many issues they tossed it after 2 days. We buy them off ebay as routine steps in theft investigations, etc. and have seen the same range of good product to real crap. So buy with caution off ebay. Your better bet is to go to a smaller dealer network storefront (example, Phones To Go (not a real store but you get the drift), or any smaller store that sells mutliple carriers and models) and get to plug in your SIM card and see how it works there in the store before you buy it. Some of these guys are pretty shady, but most are willing to deal. You also have someplace to go back to if there are issues. Good luck.
