Knowing what he charges is trivial. Knowing the other seller's constraints is what eveyone wants to know - how much longer will he survive selling at that price? how is he able to profit at that price? etc.
But the price is no secret. Just walk in the store and look at the price.
BWAAAAAAAAAAAAAAAAAAAAAAHAHAHAHAHA!
D@mn! You FairyTaxers have no idea how a business works!
If he runs his business as well as you do, you should be worried; if not, squeeze the hell out of him.
But, on the whole, you know what his employees cost (you swap the occasional employee, if nothing else), you know what his suppliers charge (after all, you buy from them too), and you know who his primary customers are (as you plot and plan how and when to steal them during their busy season). It's no secret. And, sometimes, you have an unspoken agreement about certain customers, items, territories, or products - it makes life easy and guarantees us all our Christmas gooses.
And any upstart newcomer gets his nuts profit-margin squeezed until he leaves or joins the party.