Kimber Kable Select

How do I prove that I have what I have ? Photos ? 

When you post a classified you should definitely include photos of the actual cables.  And for the sake of all of our eyes, please use moderately high resolution and a flash if indoors so we can actually see the bloody things.  Read through other ads for speaker cables so you get an idea of the content people expect.  And price them realistically.  Unless they are current generation, used cables sell at a substantial discount from "retail", whatever that is. 

Substantial discount. So discount, say... 30% from the price paid ? 

If you've never been exposed to the internet market, you should start selling/buying some cheap stuff, get some feedback from buyers and than place for sale something more serious.
 How did you find out about Audiogon?

Emojo, as has been alluded to, there are lots of scams in the audio world as there are in the rest of the world.  There are many counterfeit products, Kimber included.  Since the items were purchased in an audio store I would recommend you take them back there or at least contact them to see if they would sell on consignment. If your story is legit (not saying it isn't) then this would be the easiest for you.