The most common way to handle this is: customer gives you money, you generate a unique unlock key and provide that to the customer. The key should be a valid unlock key, and it should encode identification information about the user. Customer uses that key to install on as many of their computers as they like, or is allowed by the license.
Don't worry about reverse-engineering. Your product, if it's at all popular, will be pirated - you'll find unlock keys online if you look hard enough. If you like, you can take that as a compliment - someone liked your software enough to steal it. Cold comfort, I know, but the alternative is to get in an arms race with the pirates, and you won't win that. In the end, a few percent of the population will steal software, and you can't do much about that. You can use the key's identification information, if your scheme is strong enough, to trace who released the key in the first place.
