My understanding:
If you do a rebuild of your Remington and then sell it later, not a big deal. Its a gun you already own and your not really "building" it. It can be classified as a repair, no different than if you installed a new scope or engraved the floor plate.
Take a bar of stock material and make a receiver, build a gun on it, and then sell later; that IS a big deal in the eyes of the BATF because they didn't get their 15% excise tax or their fees for your FFL application to manufacture firearms. They'll conjure up big intimidating terms like "violation of Federal Firearms Laws" and then your ___ked for life.
Letting the neighbors use your lathe to work on their personal guns? Nothing at all wrong with that. I'd go so far as to say you could even help, so long as your not charging them money for it.
Bear in mind, that if someone (BATF) went poking around and started asking questions, all they need to do is suspect that you are doing something you shouldn't and they'll make your life hell.
Your second mistake in all this was posting this up in a forum. Now everyone sees it.
Just my two cents.
