Monsters should be privy to the same information about the world as the characters (subject to their intelligence), and should interact with the world based on that information (and their own natures).
A monster seeking a meal would probably lie in wait, try to ambush the most lightly armored opponent, and drag them off for a peaceful meal. (if encountered outside its lair). A more powerful monster in a lair may be satisfied killing one or two characters for a future meal and letting the rest of the party escape (
@Retreater ‘s HPK). An intelligent monster is actively going to try to prevent its enemies from escaping, and is absolutely going to set up countermeasures against their return (after several hours or a few days). And of course, monsters include undead and demons, for whom destroying life IS their goal.
The “I scare you off then take no actions while you go back to town and buy the items and reinforcements” is rather unlikely given the large number of monsters with human intelligence or higher.
I tend to ask myself how a party would react if the situation were reversed, and the fact of the matter is most parties I have DMed are very reluctant to simply let an opponent run away if they can prevent it.