The thing to remember with Best Match is that it is also designed to give everyone a fair go. It is designed to shuffle listings around into different orders so that the same listings are not always at the top. This is in combination with delivering the buyers' supposed 'best match' for what they are searching for (based on the words they enter and their past buying history). If different listings/sellers show at the top of searches on different occassions then it is working. The problem is that there are so many listings for the exact same thing that they can't all be on the first page of a search, so Ebay have come up with this BM as a way to supposedly give all sellers a fair go. But if there are 50 pages of listings for your item, then it is impossible to get a fair go.