On the positive:
1) Consignment stores often work on this type of loop. Bring a bunch of clothes, hope they sell, and apply the store credit to new purchases. Well-run consignment stores optimize to minimize their net cash payouts.
2) There are definitely clothes shoppers who feel they should reduce their existing inventory before purchasing new items.

Challenges:
It's likely though that by trying to force people into a behavior that may not satisfy their desire in that moment, that you end up creating more problems for the user and yourself. For example, you might lose a seller of great items that would sell quickly, because you're forcing them to buy something and they can't find something they love.

In commerce, it should all be about reducing friction, and the more barriers you put up, the more steps required to complete an action, the less success you're likely to have.

Here's an idea though: Why don't you just create a form of "loyalty point" or simple reputation system where people who keep up a minimum purchase quantity on your site become power users, where their listings are given higher priority than non power users.

Who knows? You might find that sellers who won't use your site to buy other clothes see the power listing and ask to pay you directly to access that feature.

Bottom line, I think you can achieve what you want to do without making it feel as forced as you initially describe.

Happy to talk this through in more detail with you.