For me, it's about looking for Iocals to trade with. I don't want to put my games up for sale across all of Pinside, at least not at first. Too many people want to ship games; and I'm really one who prefers face to face, cash on the table, after the buyer has a healthy opportunity to see both the good and bad of the game. I'd rather sell it cheaper, locally, than deal with shippers (time is money, especially on business days for me), and deal with buyers who don't see the game until it lands. Who needs the drama?
I'd rather ping a few people actually looking for the game, and if nothing bites, use the local for sale thread; and if still nothing, only then resort to a global ad.
It isn't about fees, either. When I sell to someone on pinside informally, I'll post the game, and then mark it sold immediately, pay Robin his cut, and also solicit for feedback from the buyer. But, I could see how many people would avoid paying fees that they don't have to.