There is nothing in our terms to prevent you from charging for your services for a site built with Yola. Any arrangement you enter into with a client is entirely between the two of you. Yola will not become involved should any dispute arise regarding this.
We would only urge you to create separate accounts for any site you are creating for someone else, as there is no way to transfer a site from one account to another once it is created.
We also recommend that you be very clear with your client that they will need to select from a library of styles available in Yola and that the customization options at this stage are limited. From experience I know that this is the biggest problem website designers have with their clients when using a free website creator.
Good luck with your business!