You're not really smithing tho'. Not much can go wrong when building an AR if you know what you're doing.
If I was running a business like that, I would stick with certain key components that I personally know and trust, like certain makes of buffers, springs, bolts, and maybe a particular line of barrels, and make those the only options. Those are internal and unseen, therefor of little consequence to the uninformed buyer, everything else they have to choose from is pretty much aesthetic and would make little to no difference in the function of the rifle.
I've had that idea myself, but figured it wasn't a good one cause it's jsut so easy to do yourself.
Meanwhile, that's pretty much what people like ATRS are already doing.