I've been doing this few times when I've cleared some random junk from my banks. Basically I try to sell most of them through the AH, but there are few items you really can't sell for any price and while vendoring them might be profitable, it's funnier to send them to a competitor. Most of the times I see them posting them to the AH for weeks and weeks.
Of course I'm losing a bit there and the competitor might actually get a sale on few of the items eventually, but the bother of trying to sell them yourself isn't worth it so at least I get some enjoyment out of them.
