Well, how many people out there actually know what the term "watchco" means? Sure, OF regulars do but an awful lot of people out there might be misled by the much more well-know terms "Omega" and "100% Authentic" and the "Seamaster 300" etc. I think the term "watchco" is actually kind of "embedded" in a way as to be easily missed, even. The lister could have easily added that the watch is made up of genuine Omega service parts paired with a vintage movement, of some such, if he wanted to be truly upfront about it all. I think it falls on the misleading side of the line, myself.
Now, how the Ebay authentication police will read all this is an open question.