I'm afraid I can't clarify too much more than the info, as it's a little beyond my scope as well, but from what I do know, the API gives access to a much more vast amount of information - data, statistics etc - about sales, performance of top ranked listings, buyer behaviour and so forth, as well as provide the ability to develop applications that interact directly with, and using, that data. (Click and send would effectively use the API to communicate with eBay and retrieve your sales records so that the data can be pre-filled. This is another app that would use the API to determine the results and recommendations in regards to titles and search queries: http://labs.ebay.com/erl/demoto/to? - the tool basically shows you the popularity and frequency of searches by the individual words in your title, allowing to to possibly create titles with optimum searchability, it was developed pre-Best Match, I believe).
So, while the data can't be manipulated the API can provide insight and direction to some of the finer details that can affect Best Match - things like order of words in the title and so on, which can have an affect on rankings, although I think it's lower down the scale of priorities and ultimately probably only has a minor influence in saturated categories. Unfortunately, I don't know or understand enough to know the full extent of how the data can be used, or how effective it has the potential to be.
On a sidenote, though, from my understanding of eBay's "Best Listing Practices", listings that have free postage, at least a 14 day but preferably a 30 day no questions asked, money back return policy and a 0-1 day handling time will outperform other, similar listings despite TRS and feedback percentage.
Sometimes, I think it's also better to 'refresh' a listing. With things like sales to impressions also having an impact, if an old listing is at the bottom of the list, IMHO there's no harm in ending it and starting from scratch, effectively re-setting some of the things that could be pushing it back.
