Your dad can leave a follow up comment to his negative. "Retaliatory feedback; item sold inferior quality". Most bidders/sellers will not think poorly of your father due to ONE negative feedback. I too have one comment from a non-paying bidder. I gave her 4 weeks to send payment (she was also from Germany...no offense to Germans...) and never did send payment nor respond to my emails. When I left her a negative comment, she immediately, within seconds, sent me one back with an inaccurate account of the transaction.

Ebay will not, under normal circumstances, remove negatives. However, if you father does work his dissatisfaction out with the seller, they can mutually remove or retract their comments.

I have to say I've looked at their items on ebay, and will probably stay clear of them. So, at the end of the day, your original post has done some good and perhaps, the circumstances with your dad will affect their ebay business in some small way.