Discount hunter is right, latex paint will work, multiple coats of it though, but for a walnut you are better off going for something better and that is the Anchorseal. I mill with both a bandsaw and a chainsaw mill. I have had walnuts offered to me before for silly prices and the little cartoon video is bang on. The wood inside might not be any good so I myself would not risk my cash on the unknown.
Sometimes I will work out a deal with a person that I will mill the log, I get to keep half the boards and they can keep the other half. You would get more for the milled boards at the end of the day but you would have to find someone that would work a deal like that. If I lived closer I would probably offer that.
I think that it would be much easier for you to find interest in purchasing the milled would rather than the log. At the same time any miller would probably be able to tell you how best to stack it properly until you sell it.
Good luck with it.
