If I was in your boat, I would look at doing the "regular" or "prolific" sellers AND buyers for free, how you determine which ones fit this is your call, but if a several hundred verified badges appear, people will notice them.
Then slowly roll it out, so everyone else gets it. It will create a system not unlike ebay feedback or any other score system were unverified members sell for less, and verified members being trusted earn more. Which in turn will push the legit users verify, offer a free verification where they upload proof to their account, or a paid version via a trusted 3rd party.
You'd have to think about passport/drivers license, when we enforced this policy on certain club nights (like named tickets only) lots of people didn't have either, so need to consider other items.