I am only sure about the first part of my answer: "market town", maybe there is another word they used to refer to the judge. In either case it would have been a "normal" judge with legal responsibilities, nothing to do with 'market' as in the business use of the word.
See also this ling:
http://en.wikipedia.org/wiki/Market_town

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Note added at 1 hr (2008-01-18 13:36:49 GMT)
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link

Duties included assigning stall locations, inspecting goods to protect buyers from shoddy materials, and ensured that the fees and rents owed the monarch were collected and delivered.

"Court cases involving market transactions were handled by a market judge with specialized knowledge of trade matters, who also made sure that the sovereign received the rents and fees to which he was entitled." Jean W. Sedlar, *East Central Europe in the Middle Ages, 1000-1500*, "Towns and Townspeople" (UWashP, 1994), p. 134

"In 1785, Perchtoldsdorf – like all small towns in the country – received a "regulated municipal council". Instead of the market judge it was now a mayor who headed the municipal council."
http://www.perchtoldsdorf.at/english/history.htm


Market, in the since of towns receiving the priviledge to hold a market once or more times yearly, did accrete the meaning of a town larger than a village but smaller than a town, but the Marktrichter was not originally related to the town designation, but to his activities in regulating the market, the merchants, and their merchandise.

or judge in the market court.
Haberkern/Wallach's Hilfswörterbuch für Historiker cross-references this term to Marktgericht, and there the definition is as follows:
seit karol. Zeit bes. Gericht in Marktsachen, wo durch Marktrichter bei Marktbann gerichtet wurde, ... in Deutschland meistens in Stadtgericht aufgegangen.