In order to answer your questions, we need to see pictures of the knife and how you had it clamped. We need to see where the scratches were on the blade/edge. I can’t tell you what you were doing right or wrong with out more facts and visuals.
As to replacing the knife, that is between you and the knife owner, but it probably was not a wise decision to sharpen a knife for pay until you had mastered the rig . Although the learning curve for the WEPS is not steep, there are skills to be learned preferably on your own knives first before trying to do it ‘professionally’, that is in this case for pay. People’s knives are special to them and one must approach each blade differently and this requires some broad experiences. Hopefully, your customer will be understanding.
Put up some pictures of the knife and its postion in the clamp and we can probably be more helpful. I suspect that the blade has a deep belly and you may have placed it in a less than optimal position. It would also help to know if the blade was full flat ground or some other difficult blade geometry.
If you are going into business, it is essential to clearly lay down some legal decisions as to warranty and liabilities and the responsibilities of the customer and those of the sharpener. A few bucks to a good lawyer will pay off in good dividends for you, instead of trying to set up your own. Legalize can get quite complex so that you think you are protected but you really are not.
I hope this helps in some way, at the least show us the pictures of the scratches and where you clamped the knife with the actual knife.
I caution here about the use of the word professional…at its simplest it means doing something for pay, but it’s true meaning is know the craft you are carrying out with wide experience and knowledge and charging a rate commensurate with your skills.
Leo
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