interesting. Bremerton CL posting is instructive too -- may be a good motor, but it's a '58 on the face of it. will run in a bucket, he says. Kind of typical find/ad -- could be a good motor. There is a buyers learning curve (upping the ante a couple hundred dollars isn't going to do much -- can spend $2K on a used motor, and still have to know what you are doing). Maybe best to buy from a dealer the first time. That '56/'58 takes a pressure tank if original, btw -- fine with me, but might be a surprise. If you bought a running early to mid '60s motor, you might be more comfortable (get a new plastic tank).
When you go shopping, think I would start with a dealer or marina and see what they have first and ask questions.
[late summer/fall is a good time to shop.]