I'm a little bit nervous about your strategy of making everything (stock, forend, grip area, etc) oversized and then relying on customers to customize it.
I'm a pretty DIY guy, but I get real nervous cutting on things that I spent more than about $50 on. For $500, I expect to have a usable, premium product out the door.
My guess is that you're not going to refund/exchange for people when they accidentally slip with a dremel and cut a groove somewhere it's not supposed to be, or cut their stock on an angle because they have a crappy table saw fence, or cut the rail mounts off their forend because they measured and set a stop block wrong.
Not to mention the fact that if you are cutting and sanding, you're going to have to refinish.
I would suggest having 2 versions - an unfinished DIY version (maybe for a bit less $) and a totally slimmed down, finished version.