Usage of a web application that I sold for an academic project: Copyright issues? I am currently a master's student and really glad that I learned almost everything about plagiarism prior to my master's thesis, I really want it to be perfect.

Anyway, I have an issue that has been lurking around my mind for a while, it wasn't a big deal before but I am currently making a huge moral deal out of it to the point of not deserving my BBA!

It is regarding my senior (or BBA graduation) project, I used to be a web developer well before taking any university courses, so when the time came for the senior projects, I was like: "Hey, I have that excellent web application (I worked on its codes for a month!)  which I developed and sold 2 years before to a local organization, why not use it for the senior project?". I earned "very good" (a grade between 14 and 16 over 20) on it.

Did I infringe any copyright here since I already sold the web application to another party? Back then, I thought of this and since I wasn't a perfectionist, it was okay for me since I am ONLY using it for an academic (and not a financial) purpose. To clarify, it went like this: I developed the web application for the other party, they paid for it, I taught them how to use the web application and that's it, I never gave them the source code as I remember (this was 6 years ago). I discovered a few months later that they aren't using it anymore and went for another web application by another developer. One of the answers makes sense, I made the source code available for public (other students) when I submitted it for my graduation project, I feel guilty now that I did not think of this in this sense back then. What can I do to for making up? Does this affect, in any way, my credibility of holding this BBA?
After your clarifications, I don't see that from the point of view of the company, you did anything wrong. Since you sold them the application and not the source code and since you didn't transfer your copyrights to them, or signed any contract saying you will not resell/reuse the app, there is no issue.

From the point of view of your university, it really depends. You reused previous work for your project and you will find people arguing on both sides of whether that's ethical (check Is reusing old code for a new assignment considered self plagiarism? How to protect yourself if you consider it to be, and a group partner does not?). In my opinion, it is not the best thing you can do because you are supposed to learn something new in your master thesis, but personally I don't find it unethical. 

Moreover, your supervisor(s) must have realized it when you showed up with a finished/polished product, with a designer on the acknowledgments and without asking him/her any questions on how to get there. If he/she didn't realize, then he/she didn't really care.

If this is really eating you up, you can have someone else (maybe a current student at the same institute) send an email to your past supervisor and asking if it's OK to use material from previous work in ones master thesis. In any case, don't beat up yourself.