Knives, honestly I’d probably just do them here. You are more likely to find people who know the value vs some random Joe Schmo. Or send em to ACKs and let them do it all and take a cut.
Firearms, I would say it depends. Are you talking things like Knights Armament and Staccato level or Hi Point level?
Some shops will do stuff on consignment for a cut of what they sell for. Others will buy outright but you will get a lower price since they will still need to sell them either in their shop or list on GunBroker.
It also depends on your state and what their laws are, but you could also do private sales, though that also introduces some danger.
High quality stuff will mostly retain its value while other brands won’t be worth a whole lot. The market is pretty strong for rare stuff if you aren’t in a rush. For instance I saw a Knights Lunchbox rifle go for over $12k and it cost less than 4K. But there were only 149 of them made.
Ammo, yea not really gonna sell that u less it’s to your buddy on the cheap.
So again, it just depends on what you have and where you live