You certainly can, and that's how most of the newer collectors are aiming to complete their collections over time. Just bear in mind the following first before you go out and spend your irl gold:
Only buy from a reputable seller who has good feedback - Unfortunately there are many scams out there, so keep your eyes open and remember if it sounds too good to be true, it probably is. I would only buy from someone who has solid feedback and provides insurance like on ebays guarantee. This only works when you have the physical loot sent, a code is not covered!
If buying for a trade ingame, again find a reputable seller, there are several on here that have made successful trades, but if you have no feedback expect to go first as they have earned theirs. This is all at your own risk, though I like to believe issues are very few and far in between.
Make sure you buy the correct region of the game or the code will not work - I've heard of a couple of friendly GM's that might try switch it for you, but tbh it's not there job and it's highly unlikely to get help with that anymore.
If you are in EU (which you are) the TBC CE is NO LONGER AVAILABLE FOR COLLECTION. Previously you could prove proof of purchase but Blizz no longer assist with this and the code WILL BE USELESS
Finally, although we are a trading site there are certain pets that Cannot be traded here
due to the value and risk associated.
Good luck I hope you are able to track down some of these lovely pets, just be cautious on buyers and remember WarcraftPets takes no responsibility for the outcome of trades made here.