How do normal vendors know at what level to set their selling prices when times are normal? They look around to see what competitors do, look at their costs, and engage in a trial-and-error process. Where it all starts? We don't know that for the normal times, how are we supposed to know it for the abnormal ones?
What is different in hyperinflationary times? That a wrong "guess" can have devastating consequences. So vendors intensify their information gathering search, they change prices daily or even, more than one times during the same day, and also (I would venture), in order to guard against the devastating consequences, they tend to set their prices even higher than what a "rational" processing of information would suggest (since they know they face great Knightian uncertainty rather than merely "risk"), fueling hyperinflation even more...
...and then, many of them still get it "wrong", and suffer huge losses as a consequence.
PS: I cannot resist the temptation to link to the fascinating paper by T.J. Sargent "The Ends of Four Big Inflations" (1982), (freely downloadable here). It is only very indirectly relevant to what the OP asks, in that it provides ample data that show how fast inflation rose in the four countries examined (Austria, Hungary, Poland, Germany), during their hyperinflation incidents, and so gives a feeling of what where the time constraints under which vendors had to decide on pricing... but it is relevant to the general subject of hyperinflation of course.