If someone has a villager in boxes ready to move out, someone from another town can talk to that villager to make that villager move to that person's town. To sell a villager means to make someone pay for the right to talk to that villager to get the villager to move in.
When u buy a villager u want from someone else in the villager trading plaza, or u have a villager u don't want and someone else wants them so u give them the villager for a villager u want that they have or igb or tbt or other items. Hope this helps.