As far as I've read and heard, guest race goods don't really have a use outside of the chapter they were gathered in... with the sole exception of being used as building material for roads. Up until chapter 9, that is. At that point, that's where it then uses other non-guest goods to build and remains that way for the remainder of the game (which is, as of this writing, up to chapter 19).
You might also want to hold onto those buildings for their sale value. They store quite a bit of coin/supplies in building form, and building the next settlement's buildings can get real expensive, real fast.
As for the actual chapter you're moving on to, it's tricky like the fairies themselves. If you feel like getting spoiled on it, check out
this guide for completing that chapter. Do keep in mind that the guide was made before Portal Profits were a thing. Read very carefully what the spell is going to give to you before confirming the cast; you'll find that you will not receive
any of the uncapped goods. And if your portal isn't a high enough level, you won't receive one or any of the capped goods either (as the portal cannot store any at that point)