A few things bear looking into:
- "carbon stainless" barrel??? Is it stainless, or alloyed carbon steel?
- "Rhondo" (Rondo?) mag well - unnecessary
- 30-round mags are about $15 each now if you pay top dollar
- laser bore sight - for sighting in the red dot? why would he not keep this for other rifles? I don't know a soul with a red dot sight on an AR that bore-sights them... not with 4 MOA dots.
- red fiberoptic sights? not a real "red dot" sight?
- McCollough compensator??? can't find that anywhere. Sure don't need a comp on an AR for recoil reasons.
Good, brass-cased ammo is about 40 cents a round these days, the Wolf and other steel-cased ammo about half that.
Bushamsters are decent hobby-level guns, better than DPMS and Olympics, but well behind Colts and the S+W LEO carbines. If his gun is in pristine condition I'd say it's worth $600-700 tops with the extras (which have dubious value). If the ammo is US-made, brass-cased stuff, $1500 would be a very fair offer, $1100 if the ammo is steel-cased.
If he rejects your offer, search for the AR shopping guide in this forum. A sharp shopper could find a good-quality AR for around $1000 today, then work on getting the extras you really need and can use.