i don't understand. if there is already a "monopoly", why bother to have a price list in the first place? it's not as if you're going to be able to get the same product cheaper or more expensive at another shop.
anyway, as mikemann said, total transparency may not be good. i bet these music shops could easily have posted their prices on their websites, but they have not. more often than not, their websites tell u to "come down to the showroom for more info". my point is, the last thing these shops want to do is to drive potential customers away by posting their prices. whereas if they have customers comin down to personally check out the products, the sales staff still have the opportunity to try and persuade doubtful customers, or offer slightly lower prices to those who're really interested to buy but is just a little out of their budget, etc.
price lists are good if there are a large number of products moving on the market at high volume, among a large number of competitors and buyers, and where prices constantly fluctuate. such as in sim lim square. having price lists saves time, makes selling and buying more efficient. but does all this apply to music gear? i don't think so.
as for trying to nanny 2nd-hand buyers, please give yourselves more credit than that. in the internet age information is so readily available. laziness is no excuse for ignorance. be thankful u do not have to make long distance calls to company helplines to inquire selling prices of certain products in year 19XX. everything is just a simple email or google search away nowadays.