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This question comes up often here on AAPK, and there is no single answer.
Just to get rid of rust from the blades - use the lead of a #2 pencil to scrub off any active rust. Some people follow that up with 00 steel wool and WD-40.
Others sand and polish, with various methods.
Then there are several different methods of cleaning down inside the knife.
Do a search for "cleaning", and you will find several threads dedicated to it, and dozens of opinions on how little or how much you want to clean.
Ultimately it is all about what you want to do. Various cleaning methods may add value to your knives, or may drop the values. There is not a "one size fits all" answer.
As far as selling knives - eBay is good. Also contributing members to AAPK can sell here on this site.
Jesus is life.
Everything else is just a hobby.