If you know about cryptocurrency (Bitcoin, Ethereum and Litecon) and can get it at its current rates, that's a winner to sell. Gold and silver coins sell as well for me.
Other things that sell well are video games and movies, both on discs and just digital codes.
Also, household staples, toilet paper; paper towels; dish, dishwasher and bar soap; and clothing detergent used to sell well for me as late as last year, but aren't doing so well for me now. The secret there might be shelling out for the extra Prime Pantry subscription (in addition to Amazon Prime) which then might let you undercut other sellers of the same items on Listia, but then you have to factor in the extra cost of Prime Pantry, in addition to Prime.
Of course, gift cards sell well too, but it might seem funny if you are saving up for one by selling them...
Look at the 30-day leaderboards to see what the current leaders are selling: https://www.listia.com/leaderboard/positive_seller_feedback_received_recent
Keep in mind that experts say, sell what you know. I don't always follow that advice, but think about your hobbies and stuff you know about, and try to sell that stuff first. Also, of course, junk you have around the house you are not using especially new things you got as gifts, you never opened.
Also, if you weren't aware of it on Listia, you don't need to own the stuff you list. You can "drop ship" it. That is when someone buys the thing from you, you order it and have it shipped directly to the buyer. Shipping is a significant expense and Amazon's free shipping with a Prime account is useful if you order a lot of stuff for yourself anyway. Amazon lets you ship it to other addresses no problem too.