eBay's policy states that the laws of the states where the buyer and seller reside are what matters to them. However, that only has to do with the sales transaction. Florida law prohibits any re-sale of partially used tickets (wholly unused tickets are fair game). So, it's legal for me in Michigan to buy a PH ticket with 2 days left on it, but if I take it to Florida and try and use it and the transfer is discovered, then the ticket can be taken away from me. The chances aren't great of that happening, but recently there have be reports of people selling restricted tickets on eBay that require ID (such as discounted military tickets) and people getting burned at the turnstiles.
Personally, I would never buy park tickets off of eBay... here's why:
1) You can't know for sure if they are any good until you try and use them.
2) The "discount" you usually get off of eBay is often only 5%-10% of the value of the ticket. Given that there are legit ticket wholesalers out there that can also save you money, WHY would you want to risk that much money for such a small savings?