If you decide to cancel, the only legitimate reason would be "out of stock" which would record a defect to you. You agree by selling on eBay to be bound by a sale the same way the buyer does.
First time buyers can be scary, I totally get it. How much is the item? To me that helps me decide. If it is a lower value item (say $20 or less) I would just ship it and rely on Karma to take care of me. If it's over that I would suggest answering the buyers questions and expressing your concerns. Let them know you are trying to be honest and that you are not some big company selling items but an individual just like they are. This may or may not make it better, but I find honest and open communication in a professional manner resolves the vast majority of issues I encounter.
