Good idea. However, if the means of distribution is over the Internet, our fellow AM (anonymous monk) can work with a company such as http://www.digitalriver.com
to handle the purchasing and electronic distribution. In exchange for a portion of the sale as a commission, Digital River will set up your online ordering and notify you of sales. With my employer, they can provide our software product for download and automatically assign a serial number and provide it upon confirmation of the purchase. At the end of each day, Digital River provides us with the info on what was purchased, and we input that into our database manually. I would recommend a setup similar to this.
For a demo user, I would make a separate build of your script, and license it "FOR EVALUATION ONLY". Water down the evaluation version if you so desire so the customer has an incentive to purchase the full version (a la "lite" version). Keep a separate log of these downloads, and support these people for only a short period of time, then if they call or write back after the evaluation period has expired, then either refuse them support, or offer them the full version.