I get an email from Collector's Weekly, There are articles that talk about what is collectible, values and things like that. We also signed up for an email from Collectors Weekly that shows you price realized on advertising items.
Jewelry, even costume jewelry, even costume jewelry, usually sells strong. Look for gold or silver content marks (.925 is sterling, gold is a little more complicated, so you might check any jewelry you have questions about with a jeweler) On anything that you planned to sell check for a signature, maker's name, initials, because anything signed is more likely to sell. There are items made especially for collector markets and they don't hold their value. Walk through a vintage mall or go to a few auctions to get a sense of what the market is like in your area. There is an auction site you can access, and find licensed auctioneers in your area. If you go to auctions, try to notice how the lots are handled. We loved going to one auctioneer because the prices were low, but they were careless with the things and there was breakage. We always joked if we got an auctioneer to sell our stuff, we'd try to get the guy who got higher prices for the items.
You can try Facebook Marketplace, for either shipping or local pickup. The buyer always pays shipping. Or used to. eBay and Amazon are 2 of the many who use free shipping as an inducement to buy. There aren't any fees on Facebook Marketplace and it is good for selling big items, like sofas.
My husband and I bought and sold while we collected. We had a lot of experience in spotting a bargain or negotiating a decent price if we should something, Maybe you'll like the wheeling and dealing and start a second career One of the auctioneers I know has a shop, and is a lawyer, too. Another auctioneer runs sales and competes in bass fishing tournaments.