Just about every car you buy now until the end of January will most likely be 11 plated anyway. Even if it is 12 plated, it is still 11 built. And most places don't care what the plate says, it's the MY that counts.
thats excactly what i did. my plate says nov 09 build but its a my10 model. i got it 2yrs ago today. i took it for a drive,put 12km's on it (had 18km total on it by then), came back and said "how about selling it as a demo" which they did. i got $8500 knocked off the price. there were 7 ss utes in the yard then and they all went within a fortnight.
With or without a salesman, I'm going to drive the car hard to see what it's capable of. I drove a demo (one was new with trade plates) 5 times before I placed the order, everytime, I had a salesman with me and everytime I gave the car a hard time.
That said, I wouldn't let the valuer drive my trade in without me in the car. Nothing like a 2 x "Bankstown Boys" in a tuned Subaru turbo for a wild time. The car was smoking when we arrived back at the dealer but they were very impressed and gave me better money than I was expecting. I would not have been surprised if they farmed mine off to one of their friends wholesale.