Samsung :: Buying Used Or New GSM Phones From Ebay

I'm not new to buying phones off of eBay, but usually I do buy it now from a very large and prolific store for new imported GSM phones (like my current Samsung g600). I have never gotten a domestic US phone and want to get a Samsung Vibrant, but many of the cheaper new or used deals are from individual users with relatively smaller feedback numbers. How would I go about making sure I don't get a stolen phone from one of them? I know Verizon checks ESN numbers for their CDMA phones but I don't know what protections GSM phones have, if they have any at all. Do they check IMEI numbers? I think I remember hearing that custom firmwares will erase the number, so it isn't a good indicator and can be used against me to sell a phone. So for anyone buying domestic used or new phones, any tips on avoiding these situations? And even if I do end up with one, it probably won't brick on me like a CDMA phone would?