If you can find a locally owned, not-for-profit thrift store, that would be ideal. I’ve got one near me, it’s all volunteer run, any excess profits after building costs etc are paid go to charities. Local soup kitchens, that sorta thing. I feel good supporting it.

If you don’t have something like that near you, eBay is probably still best. If you’re in Canada, Kijiji is Canadian owned so you can feel better about supporting that rather than eBay. Facebook Marketplace can be good but you’ve gotta be careful, there’s lots of scams there.

A lot of hobbies will have used markets for that hobby specifically. Those will probably be best for those specific things. I’m not sure what you’re looking for specifically so I can’t offer suggestions on that obviously.