Urs had an idea like that a couple of years ago; it would be fun, I think. Also good publicity - the phone that you have to assemble yourself! I can see the YouTube clickbait titles now. Maybe give people a €10 discount, or have FP donate a bit of money to charity every time someone opts for a disassembled phone rather than a pre-built one?
I guess the problem with that would be that new FP3s are presumably factory-tested after they are assembled, and so selling them disassembled would involve that extra step of disassembly after testing, which would add cost. And it would probably increase the number of warranty cases as some people would screw up the assembly. Tightening screws too much, not clicking bits into place properly, getting finger grease on contacts, et cetera.