There's deals to be had on eBay..I usually look for 2,000 or more cards lots and sane shipping costs. I see a lot of guys who just want to dump newer common cards for cheap. Assuming they bought boxes/cases looking for the hits and dont care about all the common cards?
You also need to watch for auctions if they include making the seller an offer because sometimes they just want the cards gone and will take crazy low offers. I scored a super nice lot of newer hockey this way last year.
Craigslist, FaceBook Marketplace, etc. can all be hit or miss. Most of those ads are people thinking they have "valuable, vintage" cards and are asking way too much for them.
Overall my rule of thumb is a penny a card unless you can show/prove what cards you have.