How does the trading of villagers work?

[Dakimakura]

I see people posting in their signatures that they want to trade villagers, but I've never actually done it. And some people have villagers listed they actually want? How do you even know which villagers are good/bad to have?

I could only assume that you talk to your friend's villagers before they leave or something. (Also, who's Katrina?!)

 

[talisheo]

Katrina is a person that visits your town and tells you what to wear to get good luck ( to find more gold stags on the island for example) and when someone is selling a villager you go over to their town and keep talking to that villager till you convince them to move to your town.