Is there a place to sell old board games or a swap of some kind?

https://livegameauctions.com/

Anywhere else on-site at Gen Con would most likely be against current policy.

G
 

The math trade was postponed for 24 hours so you have until 10pm Central tonight to add some stuff to that.

Flea Market

https://boardgamegeek.com/geeklist/256766/gen-con-virtual-flea-market-2019/page/7?titlesonly=1