First thing you should do is to learn the market for the specific item you would like to monopolize. Let’s say it is Kingsblood. We can see a stack of Kingsblood is selling for 1 gold. There are 10 stacks on Auction House. To control the market, you have to buy out all the lots of Kingsblood, even single ones. Next move is to make auctions of Kingsblood you have bought for the price you want. Be reasonable and make it 2 gold. But all people are still being used to the old price of 1 gold and will not buy your Kingsblood at first. You have to maintain your auctions for 2 gold all the time, no need to put all Kingsblood you have at once, 3-5 lots are enough at the beginning. You can see, other sellers may start create auctions for 2 gold as well. That’s what we need. But if someone is selling for cheaper price, buy out their Kingsblood immediately. Soon people will start to accept your price of 2 gold as a regular price and will buy your stuff. Now you just to have to sell all Kingsblood you got. Control several markets. It can be anything, even twink blue items or items level 20-30 or 30-40. Same strategy works everywhere. It requires lots of time to track the markets and check auction house. It is not that easy as it may seem, you should spend at least a week of monitoring to be sure about prices, you will fail if you buy out everything at once without research. Soon or later, too many people will decide to sell your item as well. As they could see price change and think it had happened naturally. Market becomes overflooded. It is important to quit before the collapse. This always happens.
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.
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.