About the random distribution: It's the primary reason why the miniatures are so inexpensive, and it's the primary reason there's some a large selection of different models.
About kobolds on the secondary market: Yes, kobolds are Common figures, but they're Common figures that many DMs want absolute swarms of. Needing 20 or more kobolds for an encounter in D&D isn't particularly unusual. So, since demand is high, kobolds are an expensive Common. It makes perfect sense.
Me, I think the D&D miniatures are the best thing for gaming since 3E itself brought me back to D&D. I love using miniatures in my roleplaying, but I hate the expense of pewter, the weight of pewter, the fragility of pewter, and having to paint pewter. (Words cannot adequately convey how much I hate painting miniatures, despite being pretty good at it.) Before D&D miniatures, I tried pre-painted miniatures from both Dwarven Forge and em-4. In both cases, while the miniatures themselves were beautiful, the range of available figures was very poor and the sets were relatively expensive.
While it's true that the early sets of D&D miniatures suffered from, uh, uneven quality of sculpts and paint-jobs, the quality has steadily increased over the four sets (so far), and the last, Giants of Legend, was exceptional, with paint jobs better than 90 percent of the amateur paint jobs most people do on their pewter.
IME, people that rail against D&D miniatures do so either out of ignorance or as sort of a defensive response: "Must ... not ... become ... addicted ... " I can appreciate the latter, believe me.