Three things -
1. It is a Craftsman 101.21400. M6-20B is the Countershaft Bracket. The 101.21400 is the same except for Model Number and badges as the Atlas 618. The 101.21400 was in production from 1957 until 1972.
2. Actual selling of equipment and parts is not allowed in any of the discussion Fora (see the Site Rules). It is restricted to the For Sale area which will be found at the bottom of the list of Fora. For Sale - Complete Machines would probably be the appropriate one to list the lathe in.
3. Although any site member can read/browse/buy in the For Sale area, actually listing something for sale requires that the Lister be at least a Gold ($25/year) donor. The reason being that the site costs real money each month just to stay open. And none of the staff (that I know of) happens to be a millionaire philanthropist. This is all explained in the Site Rules.
The other site that you could also look into would be your local Craig's List.