Denied Test Drive on a Used car until I showed the Cash to pay for it?

Two weekends ago, I saw a Mercedes station wagon on Craigslist. It was a 2005 e320 4matic wagon for about $15000. I called the used car lot, told them the car I was interested in and I was on my way over to look at it. I am white, upper middle class, hard working, and a business owner. I was surprised when I arrived to see no cars had been moved, and it was still blocked in. I was told the owner was not there, so I could sit in the vehicle but I could not go for a test drive unless I was paying cash, and they needed to see that cash first?

Odd... but what ever...

11 days later, aka.. today, The car was still there, so, I went back, with $2000, in my pocket and wanted to drive the vehicle.... The told me until I could prove, that I could pay for the vehicle, I was not allowed to drive it? who the F carries $15,000.00 in cash? I was like wtf? I was 100% ready to buy the stupid car, but I am not paying for it first... was I discriminated against for some reason? They were both white men... Its not like it was race against race.. I left, confused as to how a Used car lot, did not want a sale?