|A medieval market [source]|
10. On market day no man shall regrate, nor sell meat, fish or other foodstuffs until the hour of prime, when the bell is rung.A regrator was someone who purchased goods in a market for re-selling later for the purpose of making a profit.
Regrating made sense for the time: a vendor couldn't or didn't always travel around. A public market was in a fixed place and time of day so that people could find it. But what about those who couldn't make it to the market? A regrator who bought up fresh fish or bread in the morning could be the only source of fish when the fisherman or baker went back home in the afternoon.
Some communities considered this unfair to the merchant (brewer, baker, etc.) who produced the original wares. The Middle Ages therefore had many laws to protect those who brought their wares to market from having their business undermined by others. For example, regrators, when allowed by the town, were forbidden to sell at a higher price than the original price.
The assizes (court) had very strict rules about as well as against regrating. In Oxford, for instance, several regrators were actually licensed. Consider the Oxford culture: half of the town population were students—younger, unsupervised after classes were done, staying up late in the Halls. The demand for late food and drink was strong, and those who took on the job of regrators enabled bakers and brewers and others to enjoy their post-market lives and not be bothered at odd times by university students. (And the town took a fee from the regrator shops.) Remember the tradition of reresoper from this post.
In a world without 24-hour diners and fast-food joints with late-night drive-throughs, regrators could be a "necessary evil."