I would sell it on a knife sales forum, but besides the above mentioned suggestions, there are also dealers that will buy your knives or sell them on consignment. Of course dealers are looking to make money, so they are not usually where you will get the best price. Instagram is another option.
To me for safety of both buyer and seller as well as price for both, it is hard to beat the knife sales forum(At least IMO). Using Std. pay pal for the protection of both.