Thank you so much, this is excellent useful feedback!
Those bugs should definitely not be happening - coins shouldn't have been showing minus signs, and I could craft equipment last time I played. All you need to do is click on the item name.
Luckily I was easily able to replicate and thus fix the bugs. Crafting was easy fix - my last update added a skill check (some items have higher skill req.), but it failed if you have not yet learnt the skill at all (from trying to craft something at least once). The coin trading was completely borked up somehow, the code there is particularly messy - will need to do a complete rewrite later... but for now it seems to work again! I tested trades in all directions with 1/10 at a time and seemed to all add up.
The city-building idea isn't in the game yet, sadly, currently there's a small bunch of upgrades for the farm. However, I've definitely still go plans to flesh that side out more soon!
Edit: Had a quick look and the shin megami tensei battle system does look quite interesting. Looking forward to watching some videos on it more in-depth later, thanks for the tip!