I can only congratulate you on understanding it. As far as I can see there is very little guidance on costs in ebay’s explanation of the FACTORS involved, or why fees should be taken other at time of sale and payment. Just a thought, do they take fee for clicks on sale and campaign fee when buyer pays.
“When you promote your listings using a priority strategy, your campaign will use a cost-per-click (CPC) model. You’ll only pay for clicks on your ads and the cost for each click is based on a second price auction, which means the fee charged will be determined by factors like listing quality, keyword relevance, your bid amount, competition from other sellers, and a reserve price (the minimum amount we've determined a click is worth, taking item quality, competition, “seasonality, and marketplace thresholds into consideration).