NEED HELP - bought phone on ebay INVALID IMEI?? how to fix?

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http://forum.xda-developers.com – I bought a brand new S3 Unlocked phone off ebay, or so i thought it was brand new. It has the screen protector and everything still on, just like from the factory. I went to activate it on tmobile and was able to make calls but no interent. After doing a little research and talking to the tmobile support guy i found that the IMIE is invalid and that the phone is rooted. I am quite good with custom ROM's (windows mobile days) and am confident in my ability to root and change the ROM. With that said, what can I do to make this phone work? This is the info on the device: Model number i9 (HowTos)