One of the interesting places that the 80% rule fails is grain. Grain is 1, and the vendors cannot give you a fractional price for a piece of grain, so they buy back grain for 100% of it's list value - 1. This may not seem like much, but if you have AK7 and some gurly drinks you can get grain for free, and quickly too.
Yeah but if you sell it to them bundled they will give you less for it than if you sell it unbundled, this is something I figured out the hard way. =) Good to know all.