Your absolute best bet is to find either a vendor or private collector who advertises that they want cards. Do a search in the wanted section of craigslist. I would say a secondary method would be eBay. I don't do eBay any more because of the high risk of being scammed out of the cards by someone filing a claim. I'd honestly rather sell them to someone locally for $10 than go through the hassle of eBay for $100.
I'd expect that there must be online forums for buying/selling cards. Heck, there are several online Lego sites. One summer while looking for a part time job, my son started sorting his old Legos and selling them on one of the sites. Made several thousand dollars doing that!