(this will take me forever to explain)
Essentially, you can have a maximum of 10 villagers in your town, right?
For trading villagers:
If person A has Julian and wants Diana, and has 10 villagers, and person B has Diana and wants Julian, and has 9 villagers, person A will have to get Julian into boxes first (the day before a villager's house completely disappeares from your town) so person B can pick him up (giving them 10 villagers)
Person A will have to time travel one day forward to get rid of Julian's house, they can then pick up Diana from person B's town.
Selling is the same except one person is giving away a villager for the other person to give them in game bells/items..etc
If you wanted to know how to pick up villagers, as long as you have 8/9 villagers in your town, you can ask another one from either the campsite/someone else's town to move in. You cannot ask more than one villager in the same day to move in.
To pick up a villager, go to the person's town the villager is in, and talk to the villager that is leaving in their house. They will essentially say something along the lines of them not knowing where they are going after they leave their current town, and the game should give you the option to tell them to move into your own town.
Yeah.