When you start selling your program, you're the only one who owns it. Some people might want it, and since you are the only one who owns it, they might come to you and accept to buy your program at your price, $15.

Let's say I buy it. In law theory, there is a principle which comes to property right, that's states that when I own something, I have the right to use it, but also to get rid of it or to give it to someone else. When you sold me your program, unless you had me sign a specific clause in the selling contract, you gave me that right to give this program to anyone I want. This is called alienation : the act of selling makes you give up some rights regarding what is sold.

So to answer your question, you have the right to charge $15 to anyone who wants you to give him your program.

But you don't have the right to invoke public force to punish someone who bought it to you and then gave it to someone else.

My opinion is that software editors should not sell their products by individual copies, but instead they should sell publication.

Say your firm developed a software which reached $1M in development costs. You want to sell it and would like to make $100K benefit, i.e. 10% profit.

Instead of trying to sell 100 thousand copies of software to 100 thousand of people at a price of $11 each, you should first publish some advert, some demo or some restricted version of the software in order to create demand and have the public realize its usefulness. Then you sell the publication of the complete program, for a price of $1.1M.

Potential buyers would have no choice but joining their money in order to obtain the software. You might for instance create a money bar on your site and explain that your program will only be published when the money level will reach the desired amount.

According to me this is the only way to sell digital data without threatening your customers of any legal proceeding.