That depends. I found Marco Polo like story a good adventure hook: create and secure a trade route for silk, which makes no sense if silk costs the same at both end of the trade route. But I am just being silly here: it's not something I'd DM each and every campaign as the adventurers quit adventuring to become merchants. That's why I have moved toward a more abstract wealth rules and the reason I find price lists "an odd things that (slightly) bugs me". I can see guideline for magic items, for extraordinary help... but when was the last time you charged advetnurers for a mug of ale (4cp) and a hunk of cheese (1sp) and had it being a meaningful part of the story? I much prefer abstracting these expanses, as suggested by the "lifestyle" rules.
WRT to the argument above regarding fixed pricing in the real world, it doesn't apply : manufacturers' recommanded pricing is the recommanded price for a single model of a single brand. There is no price for microwave oven or car, but for a specific model at most, and not not for "a bespoke suit" or "a diamond". And certainly not for commodities (as fluctuation on markets tend to show, there is no standard price for rice or wheat even today), and in the past fluctuation in the price of bread caused revolutions, so basically fixed pricing didn't work. Local guilds could enforce standard and pricing, but a local level. Even in Eberron, there are domain where it wouldn't apply. Especially agricultural commodities, as dragonmarked houses are forbidden to own land.