Do I go ahead and do the job that they purchased?
How is the buyer bad? A little more information would help me give you a better answer.
Do you think you will be able to deliver what they want? If so, yes, go ahead. If they are one of those customers that you just cannot satisfy after delivering and re-doing, etc… you can just apologize and offer a refund to avoid negative feedback.
If you cannot deliver what they are wanting, tell them you will not be able to do what they are requesting and offer a mutual cancellation giving them a refund.
Give us a bit more info and we can then give you some advice!