"Button" won't operate correctly with the conventional HTML anchor method for some browsers, most notably IE. Normally, you use button with onclick or other script, but the normal method with onclick is to create a popup window using window.open. That method using script is seen by eBay as a policy problem, i.e. they scan for that method thinking it's an unsolicited pop-up (per recent Forum report).
The workaround is to create something that looks and functions like button, but will work inside anchor.
Danno