I tried doing a search on the forum and consulted the rulebook, which says you can sell monster and gate trophies (final battle) but not how you go about it, can someone explain.

For that matter what are trophies used for in the final battle or in general

Some locations have special abilities where you can trade in monster trophies instead of having an encounter. At the Science Building, you can trade in 5 toughness worth of trophies to gain 2 clues. For 10 toughness or 5 toughness and 1 gate trophy, you can become the Deputy at the Police Station.

During final battle you can't trade in trophies for anything, but you'll need them to stay alive against Shub-Niggurath's attack.

Tibs said:

Or Yog-Soggoth and gate trophies