I checked the serial # against this database
http://www.summilux.net/numeros/index.php It says that the camera is a IIIa manufactured in 1936. Any smart forgery maker will make sure that the model #, serial # and date of manufacture will match up with a database like this, though; the fact that they add up according to this database in no definitive indication of authenticity.
This camera does not seit well with my limited knowledge of Leica collectibles. The engraving doesn't look right.
Also, the sell has an eBay track record of zero transactions. That's a HUGE red flag, given that the starting bid is $20,000 USD.
I smell a rat...