FYI: If you’re returning to WoW after an extended absence prior to patch 5.0.1, it’s possible that you’ll have more than the maximum pet limit and/or more than 3 of a kind. However, this will be the only time that you’re allowed to surpass these limits. You will have to pare down your collection, eliminating your duplicates, in order to add new pets.
Always sell all items on Auction House. If you are not sure about its value, then check the market. There are some risks that item won’t be sold and you will lose money just putting it on Auction House. Good idea to make bank low level characters and keep everything there until the weekend. All sales increasing on Friday, Saturday and Sunday. Besides, you can advertise in a trade channel to inform people that an item you are selling has been put on Auction House.
Only really to expand on Gráinne's comment as regards the 'collecting' and 'roster development' sections - which are, as said 'distinct' although they are intrinsically linked - I would like to point you in the direction of the in-game Achievements > Battle Pets > Collect tab. There are a series of achievements called Raiding With Leashes, pretty much all of which can be completed via the auction house - if you have the gold/inclination - or by completing old raids.