Yes and no.
Let's assume the new stadium has 55,000 season tickets set aside, but only 53,000 sell. If someone buys a seat years later that wasn't part of an initial PSL, then yes, the Bills would charge one.
Now say you decide to give up your season tickets a few years from now. In order to buy your seats, a new buyer would have to pay you for your PSL. So yes, they will pay for a PSL but YOU get the money, not the Bills. If I understand the terms correctly.