Joan sells turnips.
Joan is an old boar who comes to your town on Sundays at 6:00 AM and leaves at noon. She, like other visitors, can socalize with the villagers she comes across.
If you buy 10, 50, or 100 turnips from Joan, you can sell them to Tom Nook! When you walk up to him, select "Turnip prices". He will tell you how much a turnip sells for. But don't sell them on Sunday, he won't buy them. Turnips rot after a week, so on Saturday, if you still haven't sold them, either sell them or plant them! If you plant them, they have to be watered every day or they'll die.
If it's a Saturday, you don't want to plant your turnips, and they don't have a good price, you can sell them next week with this easy cheat. All you have to do is place them in your house and pick them up the next day and then you will have another week to sell your turnips!