Just keep in mind that if the villager isn't "popular" you might not get someone willing to pay bells. There's a list on that board detailing the villager popularity.
You can either sell,Trade,Giveaway, Or buy a villager.
If you want to sell one pick the villager you want to sell put in Selling (VILLAGER NAME HERE)
Either auction the villager or set the price you want Example:
"Hi,
I am selling Cheri For 1mil Bells. Or you could offer for her."
Then you exchange Friend Codes, invite the person to your town and they will pay bells once they talk to the villager in boxes that you are selling.
ONLY sell and trade to someone with high Wi-Fi ratings or they may not pay you or trade you a villager you want and that would be a scammer.