Sounds like the seller is trying to take the transaction off eBay, which would end all your Buyer Protections.
We sellers don't actually "take" PP . Or Visa or MasterCard or GooglePay.
We are registered for eBay's internal payment processor, Managed Payments.
It is MP that takes the PP or cc payment and then passes it to us.
Why not suggest to the seller that you split the transaction into two of $300 each, you pay with PP, they get their payment through MP as usual.
And then you are covered by eBay's Money Back Guarantee, by PP's Buyer Protection, and if your wife's card is attached to the PP account by the chargeback provisions of the card.
I would be wary as a buyer of using PP attached only to a bank account, because once the money is in the hands of the seller, it is not refundable, any more than a paper cheque which has been cashed.