Lets say you make a vehicle that is sold to general public. 100 customers come to you and purchase the vehicles from you.
- If you make fake vehicles, then it is cheating and fraud and you are responsible for the consequences.
- If you use genuine parts for vehicles, then it is all right.
Now, lets say, a customer comes next day and says that he was involved in an accident because of you, then you are not responsible for the accident that he was involved in. Your other 99 customers did not come with such or similar issues.
Lets take another example, a mother comes to you and says that her son used your vehicle for criminal activities and you may be involved (according to her). Here again, you are not involved.
Now, lets assume that you are a bad person and you "somehow" use fraudulent documentation to blackmail the customers into returning the vehicle without you returning their paid money. In this case, you are the wrong person.
Now, in your case, you are developing a web application which (I understand) can be reused over and over in multiple projects. Simply put, such applications can be used for one of the two purposes
- Sell socially and ethically acceptable product
- Sell socially and ethically unacceptable product
Ask yourself and do tell me if what your product is socially and ethically acceptable or unacceptable. I don't see any harm in selling a CMS or a we b application that has a shopping cart.
What your client does with that shopping cart is another matter. You job is to develop a web application that should do exactly as your employer or yourself want. Your honesty depends on the development and quality of product that you develop.
But note that if development or designing of this web application requires performing questionable or unethical activities, then it is a problem.
Allah knows best. May Allah guide us all.