Let us upgrade the Trade Office like every other building - using beans or beanstalks.
Each level adds a permanent slot (so maximum 6 slots) at the TO, allowing six trade offers at the same time. If you think this is too much, make less slots. Make it expensive enough, but also make a fair price so it becomes an alternative to buying slots with gems/gold (one can calculate this, similar to gem pit). I'm quite sure this can be balanced in a fair way that doesn't affect gem sales.
Why is this a good idea? Despite fearing gem sales going down due to this optional way of getting more slots on TO, you should rather think about long-term effects, like players sticking with the game and bringing new people in. It keeps high level players motivated to get beans/stalks and use them for something that adds what Sharpielein expressed quite perfectly:
TO upgrade adding more slots would be exactly this. It doesn't consume island space or other rare resources, it is worth getting TO upgraded for more slots and it definitely is a progress, not only concerning game mechanics but also from a motivational point of view.How about something players actually care for - something that people would want to use with (near) infinite demand, e.g. something that:
a) Is worth getting
b) Gives you some sort of "progress"
c) Does not consume a limited resource, such as island space
You want to motivate people to play your game because that is what keeps it alive. Gem sales, that's just fast cash. You want to invest in a player base, if you really want long-term profit yourselves.