Sometimes (lol... frequently) the transaction link between ebay purchase and paypal payment does not work - it pops up a message I think that reads something like " the transaction ID is invalid".

If you paid via paypal you will need to log into your paypal account and search for that purchase and next to that will be the transaction ID you are looking for.

Has it been more than 45 days since purchase ?

Is the seller still registered on ebay ?

Was the listing removed by ebay ?

If you are trying to open a dispute in paypal you need the paypal transaction number, not the ebay Item number.