The seller is trying to give back your payment.
If he has a lot of disappointed customers, it is more efficient for him to ask for your email (which is also your PP address) than to wade through possibly hundreds of his customers to find you.
Remember he does actually already have your email address since you paid by PP.
Send the email address and thank him. At worst you end up on a spam mailing list which you can then have your email supplier Block.
It would not surprise me if his low feedback is from disappointed customers who wanted the widget more than their money. Feedback is voluntary and you don't have to leave any.
At worst you end up on a spam mailing list which you can then have your email supplier Block. I'd put off leaving FB until you are sure you are not on any spam mailing lists.