Sellers can't see orders or the names of customers until they do agree to purchase.
Sellers cannot send Unpaid Item Disputes until a customer has agreed to purchase, but has not paid within 48 to 96* hours.
So in fact you did agree to purchase. To do this you clicked 'agree' on two different pages.
You can ask the seller to end the Dispute.
He will lose money on doing this (eBay charges fees for selling here), so you could sweeten this by sending the amount of the FVF (about 20 % of the amount the UID is claiming) by Paypal's Send Money service.
It is possible you could make that small payment through the Dispute itself, worth a try anyway.
You could ignore the Dispute.
In that case you will get (at least) one Strike for non-payment on your buying account. Be aware that many sellers have an automatic Block against bidders with Strikes.
Or you could pay for the toothbrush heads and have extra in the cabinet. You would be changing them every month or so anyway so you will soon use them up.
*The time depends on the location of the seller and the site he was selling on.