Here are a few:

Shopify: First 14 days free, then a transaction fee
StoreEnvy: Completely free for your own private shop, but anything that is sold from their marketplace, they get 10%.
Etsy: $0.20 to list an item, 3.5% transaction fee.

When Etsy started, listing an items was free (just a very small transaction fee when an item sold). You want to get a big enough marketplace to attract buyers, so this is a good idea.

Perhaps you can think about adding optional features for buyers and sellers as an alternative revenue stream.