</font><font color="blue" class="small">( I thought that in America if you bought something (that was not stolen) that it was yours. )</font> That doesn't mean that owning it or using it is legal. There are lots of collector cars which can not be licensed for use on US highways. By the way, if you buy your Coopers they will have to be inspected before anyone can legally put them on the road. That means you would have to make them comply with whatever laws apply to sell them. Also if you were to try and use the Mini or Cooper names without their consent, you would be in violation of trademark laws. These reasons are also why you don't see South American versions of the original VW bug running around the US. I'm not saying that you having/using your tractor isn't legal, just that it's not that cut and dried for many things.
Anyway, Yanmar isn't saying you can't sell your tractor, you just can't use the Yanmar name or a picture of your tractor, especially on Ebay. I am sure that they are after the unofficial "Yanmar dealers" on Ebay, not the individual owners. My guess is that neither Yanmar nor Ebay want to expend the resources to determine the difference so they just throw a blanket policy on the matter. Whether this is good business practice or not doesn't really matter. Yanmar pursued what they saw as a large outlet for something they want stopped, and so far it's worked.
Also, if you had a European Ford, do you think you could walk into your local Ford dealer and expect them to carry or spend time searching for parts? Most car dealerships don't want to mess with domestic cars from the era you're asking Yanmar to support.