i tend to keep em on a shelf and pull them out for painting inspiration, or just to look back at old terrain. not long back i sat and read the original ork rules from rogue trader, and realised the writing back then was funnier, and more in keeping with the ethos of the hobby IMO than now where codex creep rules
GW world
there may be a market for some old and rare codex, i cant see any harm in posting what you got and inviting bids, you might get something in trade at the least, the good thing about the hobby is peoples willingness to share