I've been thinking about this for a while and have come up with a solution that would allow players to dispose of unwanted finished goods in a manner that is both rewarding and non-trivial, therefore able to be made a part of the game itself. It would also add a trivial, but rewarding amount of in game currency to the player as an alternate method to earn, without replacing the exiting methods.

In the town, like the HQ and the other buildings, allow a space for a "marketplace"
In this marketplace, players can stock up to 4 items for "sale". These are taken from the warehouse at a rate determined by the town population, and the reward is gold coins at a rate set by the worldwide "supply" of that particular item.
Coins can be collected every 4 hours, same as sending trains for passengers, and at the same time new items can be offered for sale. Gems can be used to "speed up" the sales to collect coins, but it should not unlock the item slot (or maybe it could?) to sell another item for coins.
The market rate should fluctuate, so that chairs may be 1 gold per 10 chairs when few people offer chairs, and maybe 1 gold per 50 chairs if everyone is selling them. The number of players world wide compared with the total number offering that item gives a supply ratio, and this supply ratio can fluctuate, but is fixed at the time of marking the items for "sale". It would be known as the 'market price' and expressed as a gold per 100 figure, (or similar).
The sale rate should be based on the number of people in the town. This makes the supply/demand rate scale correctly with the game as level and player progress continues. When the player selects an item to sell, the game client should be able to calculate how many will be "sold" in the 4 hours if the warehouse does not run out. This number should be shown prior to confirming the item going on "sale". If the warehouse runs out, the gold reward is based on the number sold prior to exhaustion.
After 4 hours, the player can collect the reward x ratio x total sold, for up to 4 items. This allows players to make more use of the factories that produce finished goods (e.g. furniture and textile) while the factories that produce secondary resources like timber and steel are kept for feedstock. It also ads a new element to the game, without compromising the existing "economy" for gold and gems.