I really LIKE the whole condition key idea. It's clean and simple. Thing is, not all buyers will see it, or even click on a link to see the definitions. However if there is a way to add at least a banner to the main page it might help. And a link in each listing next to the condition would certainly help...but because of multi-item ads ensuring the addition of a link to the condition key exists might be just too much overhead.
I sense enough people here are eBay vets in buying, selling or both. I have been lucky in that I've never gotten screwed by buyers or sellers on high dollar items (low dollar items, well I simply won't sell that sorta stuff anymore...but I digress)...anyway, buyers do not click through to get more details. Oh, some do, I know I do, but too many never do so...I am sure Adam knows just looking at the click through numbers for the site info pages. That is sort of my rational for suggesting a summary type banner on at least the first page of the marketplace, if not every page you call up...
Just a thought...I know it's a PITA to make changes to layouts and something like this might be over the top...and odds are I am just over thinking and seeing the problematic side of these things...

Gotta go get my lead lined "joke strap" in place and my tinfoil beanie before I can get some sleep.