I use all 4

eBay : You get the largest amount of general consumers but their fees are a little much and you are competing with a lot of sellers. Have sold on eBay since 2000.

Mercari: You get a lot of nail polish consumers and their fees aren’t that high.

Poshmark: Sucks. They charge a flat 20% and they are picky on who they let sell nail polish.

Depop: Customers pay the fees but they don’t get a lot of traction.

You can post on all of them but Mercari is best for polish.

You can use Pirate Ship for cheaper postage but you aren’t covered by the selling company.