I've never taken an economics class, so some of what's been said went straight over my head. Demand-pull inflation and cost-push inflation? Whosy-whatsis? Don't worry, I won't ask for an explanation, dktc - save yourself the headache! My point is that what I'm about to post may be repeating some of it in different words, for all I know. Like you, John, this is my first round and I started it late. So I'm learning the ropes as well. I have no doubt things will look quite a bit different at the beginning of the next one.
Regarding the norm selling price, I don't think there is anything that I sell anywhere near that low, either in shop or market. That surprised me as well. The only exception is milk, which I don't normally sell at all; when I do have to get rid of some, I generally have to price at norm or below in order to move it on the market. As dktc pointed out, the SuDe can be misleading in regard to determining the best shop price - some of those items are also used in the production of other things, which skews the supply side. I read somewhere that the norm price is static - only the high and low prices change, which explains why sometimes the low price is higher than the listed norm.
I've seen posts saying that buying from the market is an important part of the game. But for any product or material that I could use, the prices have been far too high to make it worthwhile. I have to assume, as you do, that it's partly because we came in late. I also suspect that there are more reasonably priced items moving through the market too quickly for you and I to see them. My own batches often sell within minutes of landing on the market. I had three of them sell out yesterday in the time it took me to refresh a page. I kid you not.
Some people, of course, buy with the intention of reselling. Others have dumped buildings producing lower-level things in favor of high-profit items, and buy to to pad their shops in order to keep that shopping value high (dktc also pointed that one out the other day). I've also read that sometimes players use the market as temporary storage of a product, and they'll price it sky-high in order to keep it from selling, which explains a bit of the market oddity we sometimes see.
On a different note, take a look at cotton. The SuDe shows that supply = demand. From that, I would expect to get a decent price for it. But the price I get when I do manage to have extra is astoundingly high, considering how little it costs to produce. But products made from it sell for quite a lot. Couple that with the fact that farms don't produce a great deal of it even at top upgrade level, and I imagine that's why people are willing to pay that much more for it.
I think the 100 building limit, coupled with upgrade limits, gives the game an interesting dynamic. Personally, I didn't expect to reach that ceiling anywhere near as quickly as I did. Once we reach a certain stage, it becomes a real challenge to keep increasing our profits.
How others play the game affects our own game very strongly. I think that learning what to expect at various stages in the round will be an important key. It has to be somewhat predictable; people develop methods that work best for them and tend to stick to those until they discover a way to improve upon them.