I thought I'd add to this with my own experience from when I started to now! It takes so long to get to a point where trading not only helps you, but helps other players too. One thing that's helped me is creating a doubles group with all of the pets I have duplicates with; a lot of players are happy trading older pets for newer ones, if they're the same rarity and something they really want. I built up my collection through this!
I also participated in a lot of pounds (both the site's own and other people's on the forums), a lot of quitting posts offer a certain amount of rares if you're quick, and some users also do givewaways for rares that you can enter for! When I was newer, this was a great way for me to build up pets for me to trade or keep in my collection until I had enough pets to trade off.
