Ravelry isn't a storefront, it's a database. When you add yarn to your personal stash on Ravelry, you can set the status as "Will trade or sell" using the 'stash tab' setting at the top of the page. If someone is looking for that particular yarn, yours will show up as available to sell, along with anyone else's that is marked as available. There's really no way to actively sell yarn on Ravelry. You might look into some of the groups/forums on Ravelry. Some of them provide opportunities for people to destash, like this one:
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