Joan usually visits during sunday and selling her turnips.
But I don't see any point in trying to buy turnip considering the fact they usually have a different price tag every day.
Personally, I get a lot of money with the introduction of Perfect Trees and since I have a 3DS XL and a New 3DS XL all I need to visit my towns and selll perfect fruits for Reese which is a good reason to ignore Joan and her stalk market.
Besides, price tags of turnips are not even set on sundays since Joan might visit.
Villagers may praize the stalk market, but selling perfect fruits to other towns is much better in my opinion.
Turnips can be very profitable, especially if you go sell them in other people's towns when they have a high price. That said, I don't like the risk so I rarely bother buying turnips. I only do that if someone here has a high price and is letting others go sell in their town.
I usually don't trade with turnips either because I don't like to play risky. At the moment I need a lot of money for my house and prefer to catch bugs at my island in order to earn it.
Nevertheless, I heard that you shouldn't buy turnips for a pricer higher than 100 - just as an orientation for those of you who want to try it anyway.
I am holding out for lower prices from Joan. She keeps selling them for 109 in my town, and that is too high of a probability for loss. In fact, I sold for a loss on Friday because I forgot to sell when it was a return (136) on Thursday.
Honestly I feel its completely fine to abuse it if you have a lot of other friends who also play. Last week all my friends where less than 120 Bells each, but mine were a 543. So they just came over.
Edit: but I will admit I don't make use of it as much as I use to.
I have bought 10 turnips at the lowest price with no intention of selling them. I just wanted them to rot in order to get flies for my museums. Bells are plentiful, why suffer the aggravation?
Turnip trading if someone has a good price is how I made my fortune. I currently have 13 million bells and I might do some more turnip trades (I want to be richest man ever, or as least as close as I can get). It isn't necessary though, but having been away for 2 years I want to start redoing my whole house and need to buy hybrids for my houses.