Here is the link to the original ad:
http://www.yorkietalk.com/forums/showthread.php?t=24561
Subsequently, there are a few postings in other people's ads that list the pup at $900 with deposit. In those other postings, it was NEVER stipulated that the deposit was non-refundable.
I also suggest you contact the Better Business Bureau, the local and state taxing authorities and the IRS to provide them with a copy of the cancelled check showing this person is conducting a sale.