My sister owns an "antique store" and so she buys and sells things for a living. She rents a storefront to store it all, but does most of her business online now.
She has fascinating stories about what sells and what doesn't, based on recent trends. For a while she was buying and reconditioning old cast iron pans. Then it was antique tape measures for hipsters who fancy themselves budding carpenters. She deals in figurines, typewriters, vinyl, vintage pinups, and chintzy collectibles like old comic book stuff. Her biggest profit center, though, is reselling old furniture she buys at estate sales. MCM chairs are made of money.
She says most of the shops in town have to specialize, because no single person can possibly know enough to make money with both sterling silverware and old watches. Each subcategory of popular item has a devoted following of rabid fans who will pay top dollar, but you have to find them (usually on social media) and you have to know what something is worth before you buy it for resale. She passes on loads of stuff that looks profitable, just because she doesn't know enough about it.
And lots of people will just bring her random shit, looking for a quick score, so she has to verify it's not stolen, and turn away a lot of tweakers.
It's a business, but not a very good one. She works crazy long hours, makes very little money, and doesn't have health insurance. Not at all recommended, unless you really love old junk.