The "T206 Wagner" does hit Ebay frequently but is ALWAYS a fake. Even the most beat-up example would command $300,000 or more so is rendered almost exclusively to auction houses these days. The T206 Plank does show up on Ebay from time-to-time for reasons that aren't entirely clear; maybe it is to avoid the auction house commissions.
This is a fantastic resource analyzing in clear prose the front to back relationships of the T206 set. This is a good read in general about the history of the T206 set but also a great source answering why certain cards only appear with certain backs, etc.
http://www.oldcardboard.com/t/t206/I...al-edition.pdf