I sold some "plusses" on Ebay last summer. I think I had two adult tickets with 2 plusses each. The bids ended up being higher than I expected (or would pay myself), but if the winning bidder used them for the water parks (more$$$ than PI) then they would of saved some money.
My only advice would be to make sure that the auction said that they were plusses ONLY (in big bold letters), not for the 4 main parks. I wrote that in my auction at least two different places so there hopefully wouldn't be any confusion and people knew for sure what they were bidding for. I did notice in other like auctions people were putting "cannot sell to bidders from Florida", so you may want to research that.
(I know that the tickets state non-transferable and you aren't supposed to do that, but at that point I needed the cash more than the PI admissions)