Shouldn't the prospective owner have need for at least one of this batteries?
I am assuming you have chargers for those batteries. You could integrate them in your solar system as they are right now; one way is to use them as backup for extended cloudy days.
While there are other ways to get energy from them, one option is to use a solar charge controller that is compatible with the voltage and amperage they output. Then that controller can be connected to your main batteries.
There are other factors that you should research, including whether or not you could keep them connected all the time. I simply don't know what systems you have and if the components will play nice with each other.