You will not be able to convince potential users that your password management app is secure and trustworthy without specifying enough detail of the architecture so users can make an informed decision.
First and foremost, you need to say what encryption algorithm is being used and the key strength. Hopefully, whatever that is, it is current and generally accepted as unbreakable against modern tactics.
You also need to speak to how your code handles the data post decryption. On the Windows platform you have the SecureString class which does a reasonably good job of avoiding keeping the unencrypted data from being stored in memory in plaintext and/or in memory longer than needed. It would not be good if users discover that if the app abends the data is in the crash dump file in plaintext.
Details of the penetration testing that has been done should be provided along with the details. Ideally, you should solicit competent penetration testers to beat up your app to reveal any weaknesses. What active defenses does the app have against common attack methods, e.g. will it detect and refuse to work if if any of the most common keylogger apps is running?
Lastly, don't forget the human element. What happens when the user forgets their key, or more likely if the app somehow corrupts or loses it's half? Is there a recovery feature? What is the process around that? If the data is simply gone with no way of getting it back, that isn't going to be embraced very well. If there is a process to recover the data, then that process needs to be scrutinized. It won't be good if I can recover encrypted data simply by finding the mother of my target on Facebook and knowing the maiden name.
Not trying to discourage, but it is very risky marketing an app of this nature. You will be competing with companies that have been in this business for decades and have the resources to hire the top minds in the world to develop and/or test the product.
Even if you could do that, you would still need a very tight legal disclaimer to protect yourself from liability. What if you woke up one morning and it is all over the news that encryption classes in .Net were exploited in a way that makes your app vulnerable? You may have done everything exactly right in your code and support processes, but you are dependent on class libraries outside of your direct control.