The current balancing system of the traders inventories seems to mainly be dependent on your stock: traders tend to want to offer or buy what you either do not want to sell or buy. I think the idea behind this, is that it forces you to diversify your economy, which is a very nice mechanic for a map such as Idyllic Village. However, when a map lacks certain resources, such as clay, this becomes and issue as the trader of who you are then dependent to deliver the required resources, makes it impossible to continue growing since there is no availability of the lacking resources.
Therefore I would like to suggest that the inventory of the trader should not only be dependent of the stocks of the player, but also should be dependent on the available resources of the map. The deep mines do lead to one problem here: they seem to be of every type on any map, which might create this issue in the first place, but these deep mines are not yet available in the early to mid game. Also, in the mid to late game, the volume of the resources you can trade are also lagging behind the economies, slowing in down to stagflation and might lead to the collapse of the entire economy.
Another issue lies in the commercialized industry one can try to build based on what traders sell. For instance, when tallow is bought in and soap is made out of it, the soap should also be able to be sold to make profit. I do understand that this has to be balanced, since it would be too easy to just create industry and buy in everything and sell everything and therefore not having to build up any economy. But it would be nice if for instance for lacking resources a small/medium surplus could be reached of certain goods such that you can sell these to make a little profit of your next step of the industry.
My suggestion would be as followed; traders should have four ways of determining their inventory:
a) The inventory should be dependent on the stocks of the player
b) The inventory should be dependent on the needs of your economy and citizens
b) The inventory should be dependent on the resources available in the map
c) The inventory should be dependent on what already was sold by the traders