To me, some of it depends on how well you know who you're sitting by. I once had to miss an ISU/iowa MBB game due to a kid's Christmas concert. Sold the tix to someone on CF at a decent price. Turns out they couldn't make it and they resold to hok fans. I had an email yet that night from the guy sitting next to me who was upset. Even when it wasn't my fault, I felt pretty bad about it, because I know how annoying it is sitting next to iowa fan.
Chances are, I'll have a couple of tickets up for sale for the iowa game. Good seats, but my wife's sister is getting induced earlier that week and she's probably skipping. I wouldn't sell them to an iowa fan even if they paid full season ticket price for those two seats. For a Cyclone fan, I'd probably sell for $200 and I will probably end up just giving them away to some friends that want to go. I get that money's money. But I'm not about to ruin my fellow fans' experience and I get a lot of joy in bringing people along (ISU fans) that don't usually get to go to games.