Is there any way to use superfluous schemes ingame other than selling them?

Schemes, like golem and building schemes for example, aren't needed more than 1 per item type and account (golem schemes are re-usable and buildings are built only once per level).

Since pets in our accounts do find schemes over and over again although there is no use for more than one per type and account, it's logical that there are, in the end, many superflous schemes laying around in inventaries and item trades. The shop for exchanging them doesn't help, since even if you do need the exchange scheme it offers, you'll find 20 next superflous schemes before you come acoss the next exchange shop.

When the game goes life, this must earlier or later lead to scheme prices around 1 silver coin per scheme since there will be too many of them in the game.

Is there any other ingame way to use them?

Or can we only hope that a sell-back option to the state store will be implemented once in future?

Not too much...

You could post this in suggestions...

A lot of sites have "trash heaps" and you can get special points, items, or currency for dropping off unwanted items (since food is so abundant that probably should not count).

We could have a special store for this perhaps? It could trade for invites or stat raising items.

You can stack the building blueprints and schemes that aren't golems. This brings the coin amount needed to craft/build down.  I tried to keep a count of how many blueprints/schemes you can use, but lost track.  I am guessing 4 or 5.  The system tells you when you can't use any more.  Once you build a building, you can't stack them anymore, but the schemes can be stacked even after you have used one and crafted.  

I have written down how much each use of a scheme (or stack) saves me.  For example, price goes down - 

astrolabe - 150 coin
astrologist 3 - 1500 coin
boat - 100 coin
etc. 

If there's any interest, I can list the amount of decreased coins for about 20 items.  

Stacking is good and all... but after that you don't have too much of a use for them...