Why? From The Torahs viewpoint, עד אלד נאמן באיסורים, so it is all the same, barring extreme negligence or dishonesty.
Including all the above responses, I had a funny conversation with Rabbi Berkovits regarding trusting private people. This rehashes some of what
@yfr bachur posted using different words and on a slightly different subject.
I came into marriage with an almost gullible trust in people I knew (not everyone, but those I knew). In contrast, my wife trusted nobody. (Really, she was paranoid about kashrus of others. When she was away from her hometown, she used to cold-call wives of Rabbanim to eat there for Shabbos, because she didn't know who else to trust.) So I went to consult with the Rav - who is right? You can be sure I brought up eid echad (they aren't selling me food, they sent a tray for the neighbor's sholom zochor...).
Rabbi Berkovits responded that we are both right. On the one hand, eid echad gives me the halachic right to trust their word. One who chooses to trust and eat cannot be told off. (Again, barring Vaad Arbah Haaratzos issues.)
On the other hand, he said, if you have no idea what they (or their WIVES) know about kashrus, you have no idea WHAT their eidus is. It is true that you can trust their eidus, but you have no idea what they are testifying.
He added: once you are no longer talking about good friends, you also do not know if they qualify as eidim. In addition, if you do not know WHO the eid is (an anonymous baker with 'kol hamotzrim badatz'), he is also not an eid. It may be that your friend knows and trusts him (and there you can wonder on what basis he does so), and so you are relying on your friend and not on the baker, but if he is as gullible as I used to be, that doesn't help.
As an aside, our home gift-delivery business is required to use food sealed with a hashgacha. Rabbi Berkovits holds that selling food without some hechsher constitutes lifnei iver.
(I have a good time with this theme some Purims. One year the poem said, 'The ingredients used were originally Badatz, but they do not also certify our pots.' Another year, I commented that the eggs and the water were not certified by the Eida, and that left for long enough, the mishloach manos may gain additional ingredients that are not under hashgacha and may be moving.)