You have to be careful buying from ebay. There are scenario where you can get a device that work now, then 2months later gets blocked by the carrier because the owner stopped paying his bills.
It's better if its a retailer with good feedbacks and accept returns. If it's possible, ask the person for the imei number so you can check it background, or ask him to check it and post a screenshot. You can check imei and esn numbers via these sites:
Also if you buy from swappa.com, they auto check imei number for all devices, however, again, it doesn't prevent the phone from being blacklisted if the owner of the phone is still in a contract and stopped paying.
You will want to get the R800a version since the R800i version will not work with 3G for ATT. I have owned all 3 versions, last I check, you can only unlock bootloader from Sony website for the R800i and R800x, not R800a. However, this website can unlock both sim and bootloader for the R800a I believe: http://www.wotanserver.com/en/sony-e...branding-flash
You basically get improved performance and more software features with custom roms. From my experience using Gingerbread custom roms like NXT gives best performance. Because lack of support from Sony and Qualcomm, ICS and JB roms aren't as fast.
You can check these guides for all the instruction with unlocking and installing rom: http://forum.xda-developers.com/show....php?t=1836530
You need to unlock bootloader, flash a custom kernel which will have ClockworkMod recovery integrated, boot into CWM recovery and flash the rom. Make sure the kernel you're using is compatible with that rom for it to boot.