Usually the 100% buy back is when you spend double on your next purchase. Another dealer here offers a lifetime buy back if you spend double. 5 year warranty is standard for a used instrument. Unfortuneatly the 10 year warranty when it was new is not transferrable from the owner to you.
I believe that dealers can quote a used price over the phone, but it is against their agreement with Yamaha to quote a new price over the phone.
I used to work for a dealer as an assistant, and I vaguely remember the new U5 going for 12k or 13k.
I guess you would want to make sure it is not a gray market piano. One not made for the US, so supposedly the instrument will not respond as well to the climate here. The serial number can determine this.
I found a website in the UK advertising a price of 5,879 pounds sterling for a new U5, this equals $11,757. Apparently in the UK dealers can advertise their prices on the internet. I don't think they can in the US. http://www.pianoplus.co.uk/index.html
To compare prices you can also search all the craigslists by googling craigslist yamaha u5. There was an 8 year old u5 for 9000 in Vancouver.
The Colorado Piano buyer's guide also says 13,5 for a new u5. That really translates to about 12 at a dealer.
See if you can negotiate free delivery and 1 free tuning.
Get an independant tech