One of the first rules of surviving the Caverns of Creation, don't fight the merchants is good advice indeed. Firstly, there's the fact that merchants are usually pretty tough customers. Well, not customers. But you know what I mean.
Secondly, there's the fact that attacking a merchant (or, for that matter, stealing from one) will cause all the monsters on that level, hostile or not, to swarm you.
But finally, assuming you survive the encounter intact or even triumphant, there's the fact that the game will not spawn any more merchants or shops, that all previously spawned merchants and shops will disappear, and that no one will buy, sell, or trade with you. The intractability of your actions really starts to dawn on you when you're hurting for a healing potion or find yourself with a million ducats and nowhere to spend them.
So, once again: don't fight the merchants.