I have a Colt 6721 Upper w/ or without bolt group. I bought it about 6 months ago and put about 200 rounds through it. I recently bought a 24" Varmiter upper and have no use for the upper.
I purchased it from Sawlesales.com for around $800, but would be willing to come down on price.
Let me know what parts you want and see if we can work out a price.