The change I made was to move the background from the full HTML page to within the boundary defined by the template. The template area does excluded the expanded left side due to the Store Search and Cat-box frames and the area below the description where Auctiva adds the SG. I also removed the <HTML> <HEAD> and <BODY> tags that might be the cause of eBay's editor deleting code, i.e. the page within a page issue. The <BODY> tag was where the background was anchored. I'd have to make a comparision of before and after change, but the evidence of your ability to change without losing the background does at least confirm it's part of that code that is dropped.
I do believe you have access to the code that includes the framed area for the Cat box with customization of the Store template at eBay, which is also embedded in the description section.
I don't know if there is a method to force the whole page with background (as before) that can survive the editor's checks. I can try using a STYLE on the page, but don't know if that would conflict. Neil/Auctiva might have a suggestion or thought on what's been done. I'll do some more checks and tests.