I've never read A Connecticut Yankee and the term "Dark Ages" is well out of date, but yes, in medieval Europe economic activities were pretty tightly controlled. There was no free market as we understand it, for land, labor, commodities, or capital. Indeed, for many times and places in medieval Europe, money was simply not used very much; serfs worked for their lords under their feudal obligations, goods were produced and distributed according to customary arrangements, and payments were made in kind instead of coin.

Medieval Europe was never really fully without money though, and when money was used for, prices and wages were controlled. The historian E. P. Thompson has argued that a "moral economy" of statutes, common laws, and customs regulated trade, and not the free market. In the case of bread in medieval England, for example, the Assize of Bread was a statute that connected the price of bread to the price of wheat, so that all bread had to be of particular, specified qualities (white, wheaten, and household were the simplified names from 1709; before that there were dozens of terms that have long since fallen out of use) and had to be sold for (or under) particular prices. These prices included a small allowance for the bakers' costs and personal profits, while the actual number of bakers in a given district would be governed by guild structures. The largest guild in England was the Worshipful Company of Bakers in London, founded in (I believe) the 12th century.

Going backward from bread to flour, all millers in England before at least the seventeenth century operated under the right of "soke." They were technically the property of the local lord (who had probably financed their construction), and all local inhabitants, including commercial bakers if they existed, were obligated to use the lord's mill. The miller took a standard "toll" from each batch of grain ground, some of which went to the lord, and a small allowance went to the miller, much as the baker got. And to go back one more step to grain, assuming a farmer actually made enough to sell substantial quantities (not always the case with the low agricultural productivity obtaining in most places at this time), it had to be sold in particular marketplaces in towns with first chances to buy going to families and individuals buying for their own consumption; only after the locals had gotten the chance to supply themselves could farmers sell to larger-scale corn merchants. Exporting grain, withholding it from the market, sales by sample, and other practices that might increase the profits associated with the grain trade were illegal and could be severely punished--though they might as often be overlooked.

These kinds of restrictions operated throughout the medieval economy, and their primary goal was to maintain social stability and continuity.