I've been selling on eBay for a while, mostly without paid advertising, but I'm thinking about promoting some of my listings for Black Friday. The problem is that my profit margins vary quite a bit between products, and I don't want to spend more on ads just to end up making less money. eBay suggests different ad rates for different listings, but I'm not sure how much I should trust those numbers. How do other sellers decide which products to promote and what percentage to set during busy shopping periods?