Where is there a source in Jewish Law that states that someone who eats a meal in the marketplace becomes disqualified from serving as a witness?
The Mishnah and Babylonian Talmud in Tractate Kiddushin page 40b discuss this topic. The text from the Soncino translation (Epstein, Rabbi Dr. I. The Babylonian Talmud. London: Soncino Press, 1935-1948) reads as follows:
“MISHNAH. HE WHO IS VERSED IN BIBLE, MISHNAH, AND SECULAR PURSUITS10 WILL NOT EASILY11 SIN, FOR IT IS SAID, AND A THREEFOLD CORD IS NOT QUICKLY BROKEN.12 BUT HE WHO LACKS BIBLE, MISHNAH AND SECULAR PURSUITS DOES NOT BELONG TO CIVILISATION.BUT HE WHO LACKS BIBLE, MISHNAH [etc.]. R. Johanan said: And he is unfit to testify.”
Talmud: “Our Rabbis taught: He who eats in the market-place is like a dog; and some say that he is unfit to testify. R. Idi b. Abin said: The halachah agrees with the latter.”
Rashi on this topic offers a reason for this law, namely that such a person acts in an undignified manner and as such lacks self-respect. Someone who lacks self-respect will not feel embarrassed about testifying falsely. The Tosafist commentators question Rashi’s reason based on other sources that imply that it is undignified only for a Torah scholar to eat in the marketplace, but for others not, and offer three other possible reasons. The first reason explains that the person snatches/steals food and eats. The second reason explains that the person goes around from vendor to vendor, tasting a little bit of each food, as though he were to purchase it but then doesn't. The third reason quotes the Tosafist commentator Rabbeinu Tam as saying that the concept refers to someone who eats a complete meal of bread (seudah) while in the market, which is considered more disgraceful. Both Rashi's and the Tosafists' explanations offer reasons that eating in the marketplace might have negative connotations for serving as a witness.