It takes 4 iron and 1 coal to make 1 tool, which makes the manufacturing cost 45, but the market price is 28-34 per tool, making it almost always worth buying tools from merchants rather than producing them. Same for some other items like furniture (cost 60, market price 60-69), weapon (cost 90, market price 80-93), and barrels (cost 40, market price 39-43).
I don’t know if it’s deliberately set up in a way for players to utilize the trading post though.
I agree with the tools, but not with the others. You gave examples, but they show that the purchase price and production price + are equal. Thus, you should not waste resources in the production of these goods.
PS sometimes merchants buy/sell swords for 120+, at this price it’s profitable to produce them
What misses in the comparison is the production time (labour). If you only compare the costs of the raw materials, you don’t have a real idea of what the total cost was.
A piece of furniture or a heavy tool takes much more time to make than an arrow. If you buy your pieces of furniture and heavy tools at the trading post, you can use your villagers to produce something else instead, so you’re saving workforce - which comes as an additional advantage.
Hence, if a weapon costs 70 to make but 80 at the trading post, might still be an interesting deal in the sense that you free some workers to produce something else instead.
There are still issues regarding items with equal costs and market prices. Like you said there’s no point producing them our own since we get no value added out of the labor invested. Some items like furniture might not even need to be bought since you can satiate the need of luxury goods by providing only candles, pottery and glassware. Thus I think the prices should be higher in terms of furniture and others mentioned above to give players an incentive to produce them locally.