I have sold unused parkhoppers twice on ebay. They expired 14 days from first use, but it ended up we couldn't stay as long as we wanted so I sold them before we left on our trip and mailed them overnight the morning after our last park day so that the person would get them in time before they expired.
Of course that was a few years ago. Now there are so many scammers on ebay that I'd be afraid to sell them because the buyer could get them, use them, then claim that they weren't any good at the ticket window, and could then file through ebay or paypal to get their money back.
If you do sell them, write down the ticket numbers so that just in case that happens maybe the tickets could be tracked to the day they were actually used which would give you proof that they were in fact used by your buyer and not expired.