Does anyone know what you do with the slots coins?

You can redeem them at the Coin Machine for some sP (usually like 60-200 in my experience, although I've gotten higher occasionally, like 800-900).

It's in Centropolis if you need to find it again in the future c:

[edit] And of course you can sell/trade/etc them like most any other item, the Coin Machine is just their "special" use.

Thanks! :)

I sell mine unless they are going for under 2000, then I just use the coin machine.