I've already contacted support about this, & was told to post in Suggestions, but I think this is more appropriate.
As it could cause problems because some of the sites are ecommerce links, so I'm going to add the name of the site, not the link.
I'm not talking about google shopping, or facebook which the Guys already do, but more focused advertising for certain types of categories.
A somebody who lists books & magazines for sale I frequently browse the web for more information on my items before listing them.
When it comes to searching via ISBN numbers, many sites give not only the book details, but places they can be bought from, eBid seems to be missing from all of them.
It could be the problem is caused by the ever elusive API, or it could be in part because the Guy's just don't know the site is there.
So a couple of book ones to start with
Fantastic Fiction
Bookfinder4U
I know over the years a number of booksellers have contacted Fantastic Fiction re adding eBid, with no joy,
but an approach to them from the site has got to be worth a try.
The following one I think will need to be paid for advertising, not a link to items but might be worth a try, (it's the sort of advertising used by the Belgian Site)
Collector's Club of Great Britain (who also publish magazines)