From Nookipedia, the Animal Crossing wiki
The player can also sell their own belongings - any furniture out on the ground floor when villagers visit is up for sale. Even if the player runs out of items to sell, the villager will take a few minutes to leave unless the player leaves, in which case the villager will leave as well. If the player leaves and re-enters the house enough, every villager will visit once. Items or creatures that a villager buys from the player will usually be displayed in their house the next day.
Flea Markets are usually held on Saturdays, but if the player is ahead of a scheduled flea market due to a time-warp, and then travels back in time, the flea market will be set to the next day (like many other events).
When the player is buying from the neighbors they can either take the price offered, or leave it. If a villager is buying from the player however, they can haggle by saying the price is too high, in which case the player has to lower it for them to buy the item.
A villager will buy a maximum of three items from the player's house. However, if the player goes into a different room without leaving the house, then returns to where the villager is, it resets so that the villager may buy another three items. This can be repeated at the player's discretion. The highest acceptable price the villager will pay is a little over twice the selling price to Tom Nook, meaning furniture will sell for half the buying cost. Fish and bugs will sell for twice the regular price as well, so if the player stocks up on high priced fish/bugs and sells them during the Flea Market, they can make double the amount of bells as selling to Nook.