Sorry for the double post. I first entered this as a reply, but later realized it probably should be a separate topic.
I'm a buyer, not a seller. By chance, I purchased a card addressed to a family member I was researching. Because I'm interested in genealogy, I now look for cards where the backs are shown or described. Prior to my lucky find, I was only interested in the images on the basis of era and locale.
Large scans are great, but a written description would work just as well. For a buyer, it's not too practical to bombard sellers with queries about who the card is addressed to (or to discover possibly some interesting feature in the message), when this could be taken care of in the item listing.
You may hear a prompt answer from small sellers or those who are just starting on ebay, but I faced myself with quite a few unanswered questions by big-time sellers who don't have the time to reply, it is undertandable since they list hundreds of postcards at a time, and sometimes buyers' queries are considered too sophisticated for them. I think the best option for both sellers and buyers is for sellers to post large-sized scans of both sides.
A quick search of the word Genealogists will show you many, many sellers who add names in their listings.
I would suggest for anyone looking for their anchestors to make sure you click on the "title and description' box when searching and don't forget to check out the stores which is usually a teeny tiny notation way down at the bottom of the page.