The scalper could "sell" their front-row ticket to someone else who buys a cheap nosebleed. Then they get in with the cheap seat and go sit at the better spot. Clumsy but it would work.
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.
This is one of many downsides of PDF ticketing. Fraudster buys a nosebleed, edits the PDF to fake a great seat. Ticket scans just fine at the door, since it has a legit barcode. (If the scanner displays the real section/row/seat for that barcode, the person working the door could cross-check it, but I've never seen that happen.)
What does this mean? That the person gets punched in the face? Do you just mean mugging people?