Merchants Guild are a type of building that can really hepl with your economy. It automatically sells all cargo you choose, and it give you an automatic sale income for every sale (by a 5% in level one). If you construct a merchants Guil in every base you have, money will be the least of you worries.
It only cost 1 ruby to construct it. You can level up the building. As an advice, I recommend you to build as much low level of this building as you can instead of few high level one's.
If you see the picture closer, you will see how it works (and the quantity of gold you can make).