Even those have price-constraints - if you watch for awhile you get a real feel for the prices that people will pay vs the "value" if you will. Certain thrift stores will be dirt cheap, others (Goodwill) will be barely below Walmart - sometimes you can even scratch the price off and find a clearance sticker for lower under it.

eBay is still worthwhile for some items, but lots of the "deals" on actual used product have moved to other marketplaces like Poshmark.

Shipping kills all of it, however.