You're providing income to someone whose almost definitely more constrained than you. Without you, no one may have bought it. And the other comment is right - it's a buyers market, we need more buyers, there's a surplus of sellers. Another thing - if sellers more quickly and more easily get to sell their stuff second hand thanks to you, they're more incentivized to sell more in the future as well instead of keeping it in a drawer or throwing it in the trash.

You're doing great for everyone involved!