Typically you're better off selling everything separately. People on this forum know what they want, and with a rifle with 5 different options on it, chances are it's not going to be absolutely perfect for anyone, so they aren't going to pay a "perfect" price.
YES! I agree completely. On various other gun related WEB sites, there are always people running ads offering tricked-out, heavily accessorized used AR rifles with hefty price tags. Those advertisement run week after week, month after month with the complete rifle unsold.
In the past, I've quickly sold off various upper assemblies and components. I always keep the lower receiver group and toss it into a draw someplace for some future whimsical project. As other posters have said, everybody has personal preferences with this stuff. The strength of the highly modular design of the AR15 actually works against selling off a moderately customized complete rifle. Take the thing apart and the components will probably move out quick. Good luck!