That's an old Benchmade 875 Sentinel, probably made around '96-'98, and ATS-34 blade steel.
I would put up fliers in the area where he found it. That is someones old trusted friend. There are likely a lot of stories between that knife and it's owner. If I had a steady user for almost 20 years, I would be beside myself if I lost it.
Forget about making a few bucks, that thing is beat up. It has lived a hard life and it shows, so it probably isn't worth much to anyone that knows about knives, and less to someone that doesn't. It should be returned to the owner with the edge and pivot untouched.
ETA: I wouldn't want to keep it. I just wouldn't feel comfortable knowing I was using someone's treasured knife, and I didn't earn it myself... Who knows, the person might be a member on here, or someone that you cross paths with regularly. Now you might have someone to talk knives with.
I would rather know a friendly/thankful person than have a beaten on knife.