We will answer your question generally, not in detail.
In cases in which full testimony is required – i.e., for monetary matters, punishments of beit din, or matters of “family status” – two witnesses are required,[2] and they must not have a direct interest in the matter.[3] However, formal testimony is not necessary for matters of issurin,[4] which is the reason why one witness suffices in such areas.[5]
When one witness is enough, a nogei’ah b’eidut (one who is impacted by the “testimony”) is also valid. One example of this rule is that a butcher is believed to say that the steps needed to make meat kosher were taken.[6] We do not suspect him of lying to make money by selling non-kosher food to kosher consumers. The certifier does, however, need to be under the presumption of reliability on religious matters. This requires that he be, first and foremost, personally observant.[7] As a rule, one who eats only kosher will not feed non-kosher food to others. Although some mainly religious people have serious flaws in their observance of certain areas of Halacha, there is a concept that we may sometimes believe such people regarding certain areas of issurin about which they are careful, while not believing them about others. One who violates “light” aveirot does not automatically lose credibility regarding “severe” ones. Sometimes, we also distinguish between whether the person stands to benefit from his testified assertion or not. (Many of the complicated details of this matter are found in Shulchan Aruch, Yoreh Deah siman 119.)
Fundamentally, there is no distinction between the reliability of men and the reliability of women regarding issurin.[8] In fact, one of the sources from which we derive that people can rely on others regarding issurin is the Torah’s description of a woman’s counting the days to end her nidda status, regarding which her husband is to trust her.[9] Rashi[10] writes that the source for a witness’s reliability in the area of issurin is the correct assumption that one may trust the kosher status of food prepared by others, and this applies to testimony of both men and women.
Given the fact that we do not require formal testimony or witnesses without interests in these cases, where did the idea of requiring hashgachot come from? The Rosh[11] writes that in his time, the broad minhag was not to trust butchers for all of the checking necessary, but to appoint experts to ensure proper standards were being maintained. Mahari HaLevi[12] posits that this was not out of fear of purposeful deceit, but rather because some elements of kashrut may be too complicated for certain butchers, who might not admit or realize their limitations.
In some communities, a proprietor who is known to be trustworthy is not required to obtain a formal hashgacha. However, most communities today require some level of rabbinic supervision. Having a mashgiach is “healthy” for the following reasons: 1) The supervision is often looser when one simply relies on the (known-to-be-trustworthy) proprietor, but since, as noted above, even honest people make mistakes, it is worthwhile for someone with appropriate training to supervise. He should be able to be diligent in catching mistakes and also know how to deal with them when they occur. In general, a mashgiach also has easier access to kashrut experts when needed. 2) One who is visiting or new in a town and does not know who is and who is not trustworthy can be guided by the certification of known rabbis or organizations, as opposed to surmising the proprietor’s reliability. 3) Every once in a while, someone who was assumed to be trustworthy turns out to not be. While Halacha does not demand us to suspect this, extra prudence on matters affecting the public can be worthwhile.
Regarding mashgichim being paid by the people whom they are supervising, there is no problem as far as the first two reasons we have mentioned are concerned. As for the third reason, however, the guarantee that there will be no corruption is indeed weaker if the proprietor can pressure the mashgiach financially to be less than sufficiently vigilant. Nevertheless, halachically, the hashgacha is still valid, because as we have seen, we do not expect trustworthy people to lie about kashrut even if they have a financial interest in doing so. Nonetheless, in order to reduce the chance of abuse of the system, many organized kashrut organizations are careful that they be the ones to pay the mashgiach.