If seller uses couriers, it is likely that they charge more to cross the water, as for all offshore
islands.
Sellers set where they will post to, not ebay. They may not even be aware that IOW is blocked.
Ebay is not going to change anything, if IOW can be blocked, then it is for a reason.
If you want to buy from a seller with this restriction, contact them to see if they will use Royal Mail and what will it cost.
This is shat sellers can choose from, as said previously it is up to sellers to use a bit of commonsense.