Locally we had a business (shut down in Dec '98 when universal registration came in) whose proprietor kept his prices low and made a straight 20% on consignments and sales.
His secret was what he called "serial gun buyers", who traded and returned the same guns over and over as they cycled thru the community.
Some guns which were recognizable (to me) cycled thru the shop 5 or more times (for a while he owned up to 3 stores in the area), netting him 100% profit on a single gun...
I guess I didn't mention the fact that he retired in '99 at the age of 40...having worked the gun business and gunshow circuit since he was 16.