eBay wants to make sales to buyers, and retain / increase sellers, so their algorithm works in eBay's favour (and possibly not in the favour of an individual seller). They spell all this out if you dig into the T&Cs as my friend did: 'To drive a positive buyer experience, a listing may not appear in some search and browse results regardless of the sort order chosen by the buyer.'
Knowing this there are common strategies that may help you work with the algorithm instead of against it. This can include promoting at the lowest 2% general rate, as you mention. I do not choose to do this myself, but lots of sellers are happy to pay 2% to help secure a sale.
