I know the price you can buy turnips at is random and the price at which Reese will buy them changes daily, but within what range are these prices? Are there any indications I can use to know that prices are particularly high?
When buying turnips, the price can be between 90 and 110 Bells.
When selling turnips, things get a bit more complicated. Looking at some data points, it seems that sell prices can vary within a huge range, but will follow certain trends. These trends, along with other tips on selling, can be found here.
In the decreasing price pattern, the prices will consistently decrease by a few bells each time, never increasing. With the decreasing pattern, the start price for the week will be between 99 and 50 bells. In this pattern, if the price doesn't increase by Thursday afternoon, you should sell immediately.
Big Spike Pattern
The big spike pattern features a decreasing pattern, but then there will be three increasing prices, with the third being the maximum for the week, followed by two decreasing prices that are still higher than average, followed by a continuation of the overall decreasing pattern. The maximum price in this pattern is always preceded by two increasing prices and always followed by two decreasing prices, so the maximum can't occur on Monday or Saturday.
Small Spike Pattern
The small spike pattern features a decreasing pattern, but then there will be four increasing prices, with the fourth being the maximum for the week, followed by a decreasing price that is higher than average, followed by a continuation of the overall decreasing pattern. The maximum price in this pattern is always preceded by three increasing prices and followed by one decreasing price, so the earliest that the maximum can occur is Tuesday afternoon and the latest is Saturday morning.
Random Price Pattern
The random price pattern features unpredictable prices in the range of about 50 to 200. The prices increase and decrease at random. The price seems to go over 110 at least twice a week in the random pattern.
By learning these trends, you can determine which day will have the highest sell prices for the week. Prices change twice a day. Once when the shop opens and again at noon. In order to properly watch for the increases described above, you will have to check every new price.