You can definitely sell your original designs on P.O.D websites as long as you are selling a flattened design where your purchased resources cannot be extracted or worked with by a third party.
Fonts cannot be embedded or uploaded to any P.O.D website, as this would allow for third parties who had not purchased a license to work with them. However, uploading original, flattened artwork (such as backgrounds, invitations, cards etc.) and allowing third parties to add custom text using the P.O.D website’s standard font options is allowed.
If you'd like further clarification, please pop us over an email with a visual example of your design, so that we can check this out for you!