If I searched for "widgets on mars", I would fully expect the first result to be the website known as "mars" and hopefully showing where "widgets" are on "mars". So I don't see anything wrong with "diyaudio on eBay" showing eBay first. You're literally asking where to find diyaudio "things" on the website known as "eBay".
If you want to find things pertaining to "eBay" on diyAudio I'd search for "eBay on diyAudio", noting that eBay has a much higher "authority" in general and would likely come up as a first result anyway as 99.9% of the population that type in "eBay" are expecting to find something on eBay itself.
To do what I think you're trying to do, you'd probably want to search for this:
site:diyaudio.com ebay