I've bought & traded knives almost every year at BLADE since it's been in Atlanta. I bought a Strider SnG & had money left over to get my Hamilton watch due to selling a Randall at BLADE.
Understand that a lot of distributors are there to make a profit, not spend their profits, so don't expect to sell something easy. It helps if what you are selling is rare or hard to get - Randalls, Morseths, Old EKs, discontinued productions (Spyderco Shabaria, Microtech Tachyon), etc. If you are trying to sell something that the buyer can find multiples of at BLADE you will be out of luck.
Additionally you can usually come out better trading towards another knife, than trying to get cash.