Can you use subdomains for clickbank sales page & Niche Websites?

Can I use a sub domain of my website to promote my own click bank product? It seems like a very smart thing to do and yet not many people seem to do it. I have not used clickbank to sell my own product so I am not sure if there is some click bank policy that is against using sub domains as url for sales page. So if I use a subdomain to put my sales page, will it create any problems for me?

Instead of spending money buying several domains for small niches, I thought why not buy a single domain and build small niche websites around it in its subdomain.

For example say I have a romance site called www.romance.com and i build smaller niche websites like say romance for the obese in the subdomain like love-for-the-obese.romance.com, will I not get the same benefits that I will get if I got my own domain for the smaller niche - romance of the obese? The idea seems brilliant because the link juice from the subdomain will flow to the main domain and I think the benefits the main domain will get from this arrangement will be obvious to warriors out here. If I had a sales page promoting my own click bank product in a subdomain, the benefits for my main domain will be stellar as the link juice will flow from the subdomain to the main domain. From my current understanding of the system works, this seems like a brilliant idea. Yet nobody seems to do it. I cant be the first person who thought of this, so I am getting the feeling I am making a newbie's mistake.

Thanks for the help