Just a few random thoughts. One thing that pisses me off and makes me happy are mislabeled listings. They are great in that you sometimes pick up nice cards for cheap, they suck in that when you are looking for a specific card it is often difficult to find the card you are looking for unless you stumble across it.
COMC has, by far the best interface. Pre loaded search terms, consistency in listings and organization. Beckett Marketplace has the same sort of set up but the searches are slow, the site often errors out, and the search results are not as good as COMC. Sportlots is pretty good, just slow. Usually very well organized and leads you to find a specific card. Sport buy sucks the worst. Very slow site, search results suck.
Ebay as we all know is a free for all. Some of the listing look like they were done by a 2nd grader, I wonder if ebay will ever go to a standardized collectible listing?