While the Resources rules are abstract, I don’t really find them to be very complex at all, but maybe that’s just me.
“Coin and cost” doesn’t cover ideas such as available assets, credit rating, all of the stupid little things people have to spend money on to survive or waste money on to keep themselves happy (do you really want to track every new pair of socks or every candy bar or every new roll of toilet paper?) and the fact that not everything costs the same all of the time. That’s how I see it, anyway.
Torchbearer is a descendant of Burning Wheel, and both are fantasy games set in a period of time when people didn’t necessarily walk around with sacks of coins all of the time (and it might not be safe to do so). So everybody keeps a “tab” for you, “I.O.U.s”. It’s like, “Hey, my harvest won’t come in until the fall. Can I get this hoe now and pay you back later?” That applies more to Burning Wheel than Torchbearer, but it’s the same concept narrowed for use by adventurers.
Think of Resources as a combination credit rating and “financial management skill”. Can I afford this sword, or did I blow all of my cash on tacos last night?
There’s also an element of “shopping” involved. How long did it take me to find the sword I wanted to buy? Did I have to pay somebody for the information? Did I have to keep up my living expenses while looking?
As for armor and rations, recall that Resources isn’t hard cash. It’s just a financial reputation, financial planning skill, and some nebulous assets. Perhaps the guy selling the rations doesn’t trust the look of the coins you have, or doesn’t want what you have to trade, or just thinks that your (lowercase) haggling makes you seem so cheap that he just doesn’t want to deal with you.
With the gold necklace, don’t think of it as a trade. Think of it as selling it off for store credit, thus adding it directly into your Resources score. Of course, failing the Resources test shows that your affluence isn’t what it looked like, despite you throwing around jewelry, and your Resources evens out again. It’s like an implied “Tax shield”.
Lifestyle is just the same thing on a larger scale.
What are you missing? I guess that The Life is hard and dangerous, and that it’s hard to pull yourself out of the gutter. Even in real life, you’d think that healthy (but poor) adults would just spend carefully and work their way out of poverty, but it doesn’t happen like that (very often) because people are people and money is money and it’s all very messy and hard to control. Yes, I think that’s it.