3.6 Million pet battles a day
900k "PvE Instances" (Raids, dungeons and scenarios) a day
Although it may seem more people raid/dungeon/etc by way of mouth, the actual numbers show a vastly different story.
Unless you have a super rare pet, mainly TCG, you won't be making a small fortune, alas it also depends on server, pets can be an excellent way of transferring money to another server.
Only pets I've bought off the AH were Darkmoon Zep & Tonk, the faire wasn't on, and I actually needed them for the celestial tournament, they were already lvl 25, they were around 3-4k each (I also bought them on another server that had them cheaper, this also impacts price and actual sales as they are account wide), and the only reason I bought them is because I needed them, anything else I've bought petwise was because it was ridiculously cheap at like 100g.
Similar to mounts, I see the Crimson Deathcharger listed by the same people week in, week out, never selling, and if you started that quest from scratch, it takes far longer than farming any of the rarest farmable pets.