Selling an unknown engine by a private party with no warrenty... Honestly I'd say asking $5k would be a bit on the high side. You're going to take a huge hit on it no matter what you do- if you have all your reciepts it'll be a little easier to sell but in the end you're selling a 500hp-ish sbc, and its going to be in the same market as other 500hp engines that were built a lot cheaper.
Try posting ads with local car clubs or at car shows/cruises.
You may be better off selling the short block, heads, and blower separately too.