Help me understand why a person would go to an event and sell their cards for 80% or less.

Genuinely curious. I have been doing some research into setting up a tabel at an event and from what I see, it's perfectly acceptable to sell your cards TO THE VENDOR for 70-80% of the FMV for those cards. I understand that on eBay or other platforms you may pay up to 15-20% in fees, but even then there's no clear benefit to going to these events and giving the extra money to other resellers instead of a trading platform.

So what gives? ​