The answer depends on several factors, but much comes down to what alternatives they have. If it's costless for sellers to join your platform, and they are able to join competing platforms (multihoming, in the lingo of two-sided marketplaces) or earn money in other ways, then there's no need for them to make a full-time living off selling on your platform. On some classic platforms--including eBay--many sellers are happy to participate as a sideline.