High-fixed setup costs, low marginal costs make a defensible moat. That's the classic case of a returns-to-scale monopoly.

Background checks are a one-time cost per new participant.

It may be costly (in money or time) to set up your initial account, and acquire a positive reputation via a long history of mutual reviews... but then you're part of a large pre-vetted community.

Do you then want to go through that procedure many times with every new upstart? Or just once with the largest market?