On the Range Exchange where plorts are sold, there's a large screen that shows the current selling price of all plorts. Just to the side of each price though is a symbol showing an arrow pointing up or down, or just a flat line. What does this symbol mean? Is it showing how the prices have changed, or if the prices are likely to change in the indicated direction?
The symbol indicates price change from the previous day.
From my experience, it shows what direction the price of the plort went form the previous day (up means higher in price, down means lower in price). I've come to this conclusion since there definietely have been days where an upward arrow was seen on a plort, yet it decreased the next day (and showed a down arrow).
At 12AM, the prices change, and the price direction is determined by how much of that plort you sell, with an additional random element, according to the Wiki:
This happens at 12 AM in-game time, when the plort market will be offline for a couple of in-game minutes. The fluctuation is based on how much Beatrix has sold with a little randomisation but they are always based on the plort's original value.
The Wiki also shows the maximum and minimum price of each plort. You can use that along with the direction of the arrow to know when it's better to sell or hold onto your plorts.
Here's a screenshot of the ranch exchange's display before...
And one day later.
The arrows simply show which direction the price has changed from the previous day, with no connection to what it has been doing in the days prior (showing a trend) or what may happen the next day.
Those arrows do mean that the price has gotten lower or higher last night. plort values change at midnight every day. usually selling lots of plorts at once can cause it to drop drasticly. of course, they will go down gradually by themseves. of course, they only change at night, so if values are high, i.e. honey plorts at 111 each, SELL SELL SELL because they can not change until midnight.