Most PhotoPost licenses are held by individual persons, in which case it is simple to understand that the license cannot be sold or transferred from that individual to another individual or from that individual to a company. Nor can that individual continue to own the license and simply sublease it out to an individual or company (for instance, if the seller were to sell their entire website and then allow the new owner to operate the seller's license of PhotoPost without buying their own). And since slavery was abolished, a buyer can't buy an individual person.
In the case of a company, companies sell individual websites all the time. Websites are simply assets, like a copy machine, that can be sold individually without selling the entire company. Again, the license stays with the company, not the website. If a company sells a website to a buying entity, the buying entity would need a new license to use PhotoPost on that website.
If a corporation, recognized as a legal entity, sells its entire assets and that entity continues to exist under new ownership, then the new buyer wouldn't need a new license, since the license holder (the corporate entity) didn't change. But if the buyer dissolves the old entity and/or merges its assets into the buying entity, then the buyer would have effectively killed the PhotoPost license holder and would need to buy a new license. This only applies to legally recognized and properly registered entities (corporations: C Corps, S Corps,
http://en.wikipedia.org/wiki/Corporation) not sole proprietorships or other make-believe or self-proclaimed organizations that are not legally recognized as a corporate entity.