Ethics question...

I have spent a lot of time working on a specific E-Commerce (shopping-cart) to fill a need for a customer. Now that the project is complete, I would like to market the product, which is entirely PHP and MySQL. My question is: Is it ethical to sell a product ( at a relatively low-cost) written in PHP, especially since this group made it possible? If that's ok to do, does anyone have any ideas how I can put a time limit on a demo that isn't easily defeated? TIA, Shane