I'm thinking the same things a lot of people have said:
1 - Almost no one keeps the buttstock so don't bother making them and sell it without!
2 - I already have more AR lowers than I need, have the option of just getting a complete upper (with the BCG and Charging handle)
3 - Longer barrel, not shorter
4 - Flat top and ditch the front post
5 - Ambi charging handle
And if you are selling the entire rifle for $600, come up with a reasonable proportionate price for just the complete upper. Some places that sell complete rifles and then just the complete upper for the same rifle are charging less than $100 less for the upper than for the complete rifle. The lower, LPK, handgrip and buttstock have to be worth more than $100???
Bundle the whole thing with some nice non-corrosive ammo (say 1000 rds) and include shipping.
Something to think about as well, uppers in multiple calibers - How's about a gas (not blowback) 9mm or .45 cal or 7.62x25
