The lack of a website or
ebay store link in their print ads probably is psychological. If you're trying to induce sellers of high quality collections to sell to you, you probably don't want them to know that in some cases you break up their collection, turn around and sell it on
ebay. They probably sell through a variety of channels including wholesale to dealers, retail to private buyers who have provided want lists, and online.
Their
ebay store has over 2,500 items, all Buy It Now, many scarce or rare and priced accordingly.