you're allow to buy/sell here, but you have to search for people selling/auctioning villagers you want if you want to buy... looking for threads aren't allowed here
alternatively lurk cycling threads and/or participate in giveaways/raffles too
tier 1 villagers though (as listed here) will almost always require dreamie proof, and I believe in some rare cases tier 2 villagers as well (depending on who's giving away/auctioning/selling the villager)