After doing a bit of research, most of the info (I was able to find in a quick search) does seem to show that the arrows indicate the price is going to increase/decrease in the future. The prices are based on real world metrics, so PD essentially is letting us know that they have noticed an increase/drop in a vehicles value, and the dealership will reflect that the next time it updates.
Learn something new everyday.
It still seems odd, though. There's not much to indicate that is what they intended (in game). I suppose it's another situation where PD be doin' PD things.