I second this. Almost every time I have had a customer ask for work ahead of time, it has resulted in a loss for me. I offer to provide examples of past work and I give a guarantee that if they are not satisfied with my work I can cancel the order and they can work with someone else. But I highly recommend not doing any work until the customer has committed with money. I am of course very polite about it, offering something like this:
I’m sorry, but I am not able to provide samples or sketches before the order clock begins. However, I am attaching samples which will attest to the quality of my work. Please feel free to check out my customer reviews as well. My work is 100% satisfaction guaranteed. If you are not happy with the order I will cheerfully refund your purchase so you may work with another seller.
I’ve lost one only potential sale due to this policy, but I can think of at least three examples where attempting to comply with a request for work beforehand resulting in a loss of money and labor hours on my part.