Only because I dabble in AR's a bit, i'd say buy it.
I'm on my 7th Bushmaster, every one of them has been a fine rifle;
Just wish I was smart enuff to realize that after I bought and sold a few to get "better" guns

Just because it said Colt, it did not shoot anywhere the Bushy did....but that is a whole nuther story.
The components of the rifle are nearly worth that of the asking price, and used or new barrells can be had relatively cheap if you choose to swap it out. They are simple enough that you can do it.
Not touting one brand over another, but Bushmaster's barrels are pretty tough, I'd doubt that you shoot one out without tens of thousands of rounds. See if you can clean the boltface, it might not really be as bad as it seems. Look inside the lugs where the bolt locks up for wear. If they will allow you, ask to see the bolt carrier group disassembled.
AR15.com always had used stuff for sale, so the guys over there could hook you up with any add ons or factory stuff you need.
If you could get them down to $600 then you would have a great deal.
Where are you in NH???