This is one place to start. You might also want to place a free classified add on the AOMCI.org website, as a lot of guys looking for old motors peruse the listings. Price is always a tough question, as condition is probably going to be the major determining factor. If in good condition (especially if you can prove it runs) somewhere around $200+ might be a decent ballpark expectation. By good I mean that it has no major mechanical flaws and the original paint and decals are in decent shape. If it is in pristine condition, maybe double that....hard to say. Take a look at the AOMCI listings and see what other motors of that horsepower are listing for.
If it looks like a junker, expect significantly less. Grandfathers tended to take care of their motors. I have my Grandfather's old Merc and it is in in better condition than a lot of motors half its age. You can always pick a higher price and say you'll take the best offer. Keep in mind that unless you find a local to buy it, shipping is going to pricey, which will affect what people are willing to pay. Might want to post on Craigslist to tap into any locals who might be interested.