I have no personal experience with buying AC day passes on eBay. None.

Here are two things that would make me hesitate:

http://www.aa.com/i18n/utility/citiA...alsclubccoffer

"Day Pass may not be bartered or sold other than by American Airlines and will become void upon such transfer. "


and

When I look at eBay and search for Admirals Club day passes, they are often photographed with the serial number clearly visible. It doesn't take a genius to figure that AA anti-fraud employees can see that serial number and void out the guest pass.

But, then, what do I know? I've never tried it. Maybe it's all OK and works every time.