You'll need to read terms and conditions but, just going on what your screenshot shows, I would have thought the code should work too. Usually when a code pops up on an actual ad, it's a signal that that item is eligible and I have never had a problem using them (although not after making an offer, I will admit).
I can see why you're a bit peeved as you'd have got it cheaper just buying at full price with the code.
Have you tried actually placing the item in the cart before you pay, rather than just trying to pay outright?
I can remember having trouble once as there didn't seem to be anywhere to apply a code, but after I placed the item in the cart, it appeared.
Apart from that, if you can't find anything to disqualify you in terms & conditions, then maybe contact ebay chat & show them your screenshot and see if there is a glitch they can fix.
