Nope, whenever it finds someone buying what you want it will sell in chunks if the person isn't buying as much as you're selling. Goes the same way with buying.
You didn't read the full post did you... He was saying how miniclip wasn't really the cause and that it had always been like that. Nice read, even if there was still beggars and such back then it was probabyl more enjoyable.
You should be fine mining iron at 15 with the best pick you can. You can mine iron to any level you want if you're looking for fast experience. Or you could try powermining granite once possible.