Buy low at garage sales, sell high on eBay
eBay is good for collectors items, I’ve sold a good amount of rare barbie dolls that my wife had collected from working at Toys R Us in the 90’s. It’s quite lucrative for that kind of stuff, I remember posting a Lord of the Rings Barbie Doll set, then sitting back and watch the bidding war shoot the price up to over $300.
Also good for selling salvage or for-parts items. If you’ve got a broken printer, chances are a refurbisher will buy it for the shell or inner working parts. Although don’t expect to get much for this type of stuff, I mean after all it IS broken.