All I sell is pets and mounts on five realms, and in my experience 95% of people who buy pets could care less about breed or level. The vast majority of the market is "gotta catch 'em all!", not pet battle min/maxers. Pet battling is too mundane for most people to care about it in much depth. I used to try to price different breeds and levels differently, but it didn't take long to realize it wasn't worth the time and storage. The number of people who really, really want that rare S/S Darkmoon Rabbit is so small that it isn't worth it to wait for those buyers. I firmly believe that almost everyone ignores breed, and they only look at level as a slight bonus.
The TSM guys decided to ignore the whole breed/level thing in their handling of pets, and that was the right call. Warcraftpets.com
is a great pet-centric site, and they, too, mostly ignore breed/spec. They show it in the detailed info on a pet, but it doesn't factor into their comparative pet collecting registry at all.