Re: Send this AR upper receiver back?
I'd be pretty ticked if I paid my hard-earned dollars for a "new" upper receiver and it arrived with boogered up threads or other "damage" as pictured above. At a minimum, I'd probably let whomever you bought it from know that it arrived in something other than "new in the box" condition with boogered threads (and any other damage that you may be able to observe) and see what they say (i.e. - they may be able to offer you a refund/exchange depending on what you want to do, how long you can wait for a replacement, etc., or if you want to keep it, they may offer you some sort of a partial refund or perhaps a discount on a future purchase/etc.).
HOWEVER...if everything screws together without issue and it doesn't effect the function/accuracy/etc. of the rifle/upper, I probably wouldn't worry about it.