Each selling platform has different fee structures. Compare them side-by-side to maximize your profit.
eBay: 13.25%. Amazon: 15% + $0.99. Etsy: 6.5% + listing fees. Poshmark: 20% (>$15). Mercari: 10%. Facebook: 5%.
Facebook Marketplace (5%) has the lowest fees, but eBay and Amazon have much larger buyer audiences.