It really depends on what you sell, and your location.
What I have done/ do:
Amazon is the easiest money. Books are a great ROI, and retail arb. is fun if you have some up-front funds.
If you are into selling locally then just look into FB Marketplace, CL, and OfferUp.
Other sites I use / have tried are Mercari, Graild, Poshmark.
I have never tried Brisksale, but heard there are zero fees.
Also remember that there are forums for a lot of different things, and they can be a good way to sell things as well ( I do this for collectibles).
After trying numerous things I stick with eBay & Amazon FBA.
Good luck on your search!