Anything goes. You can sell LEGO® pieces, LEGO® boxes, original LEGO® building instructions, LEGO® Idea Books, LEGO® comic books, LEGO® catalogs, anything. It's all been done.
Just be sure to let people know how complete the set is and generally what condition it is in. You don't necessarily have to list all the missing pieces, or every scratch or blemish, but your efforts will be appreciated if you do so.
If you're selling a LEGO® set, you'll get the most for it if you have all the pieces, the original instructions, and the original box (in particular the front and back panels of the box), but you generally won't get much more for it if it's mint-in-box than if it's been opened and played with -- as long as it's still in good condition.
Some people already have the pieces to a set and are only seeking the box or the instructions -- for example, they found the pieces at a garage sale, or their dog chewed up the instructions, or their dad threw out the box.
Some auctions even consist purely of LEGO® pieces, where the auctioneer buys dozens (sometimes hundreds) of brand-new LEGO® sets and divides them up into specialized parts packs. This is a time-consuming but lucrative business.
8 Jan 96 [TSL]