A big part of the problem is the OP didn't receive those emails.
Gencon was issuing warnings about this a good few months before it happened. They had announcements everywhere.
Indiana business journal
Every gaming group and blog were announcing it. I even actually saw it on the news at one point albeit briefly.
What did you want a personal phone call?
So I have no idea how much more they could have done. They hit every single source of information they could have. At that point, the blame can not be on the company. They have covered their bases.
They also sent out emails asking the exhibitors to get in their requests for volunteer badges ASAP before they sold out.
I mean, it is surprising that an exhibitor did not hear about Gen-Con selling out from his network, but if he is a mostly-off-the-grid kind of exhibitor and the update e-mail was really not reaching him, then the negativity is not justified.
Sure, you could wonder why he did not notice the e-mail was not arriving, but remember how blindsided we all were by the announcement that it even COULD sell out in the first place; none of us would have assumed we were missing that kind of update in the same situation.
How can my father in law sell his 4 Day Badge? They won't offer a refund because its too late. We've listed on Facebook market, craigslist and letgo. Any suggestions?