I think Bricklink and brick and mortar Lego stores are the best option. A lot of the sellers on eBay and Facebook Marketplace are usually just selling stuff they found from their kids or stuff they found. They (sometimes) aren't very experienced in pricing sets and will usually charge you a lot of money for somewhat minuscule sets. G1 sets on eBay and FM are alright, they sell for usually around 1.5x their MSRP, but G2 is a whole 'nother story. Last month, I got a sealed Skull Slicer off Bricklink for $8 and current listings for G2 on Bricklink look a lot lower than they do on eBay.
However, I do agree with you that sets do get gobbled up fast. A few weeks ago, I saw a listing (bid) on eBay for a parts lot, and in the picture, you could clearly make out a Tahu Uniter, Ketar, and a headless Pohatu Uniter. People jumped on that bid and it sold for around $40 IIRC.
I'm not very experienced on thrift stores, but I assume that you could probably pick up some G1 sets, but probably not G2, since they're relatively new and people might still want them.