Nosebleed seat means a seat at the very back of the house. Because it is usually very high up, the idea is that the atmosphere is thin and it makes your nose bleed. Basically what the person is saying is that the fan can buy a very cheap ticket and exchange it for a much more expensive ticket that the scalper bought.
I think he's referring to a seat that is not a good seat. Normally called nosebleed seats because the altitude is so high it could make your nosebleed. I think.