I see what you are saying. I guess it will depend on whether the increase in sales was significant enough, to have eBay feel it is worth doing on a regular basis. If sales tank the rest of the week on an announcement of a discount Sunday, they might then stop doing it. 

so who knows? They will have to assess the sales figures not just in isolation but for the week leading up to today and the one after. Won't affect my spending because it was purely a bonus sale for eBay.

hope it was overall a good thing for sellers after the past few months of uncertainty.