Tangential aside about the partial refunds...
I think you could avoid a lot of partial refunds, and Neutral Feedback, by disclosing all faults in the descriptions. You've been around the eBay block in the last 20+ years so you probably know that, these days, 'sold as seen' or 'see photos' doesn't really work anymore.
Personally I won't offer a partial refund. Rather, I insist that Buyers open a Return Case. It saves a lot of time messaging. It also flags up problem buyers for eBay. If I know I made a mistake, or there is a factory fault that was unknowable (e.g. inside sealed packaging), then I give a full refund without having the item sent back. The rest of the time with perfectly as-described items, most Buyers don't bother to open a Case... probably because they just wanted the item + extra discount.