Flippa used to be good, but your best bet, if you truly want to be a legit seller with a high chance of repeat business, is to go on Reddit.
Look for threads where people talk about the problems that your program solves.
Get in touch with them.
Become part of the conversation.
But whatever you do, get in touch with people who actually need your product.
A lot of the older platforms out there like Flippa can help you.
But expect a lot of competition.
If you want direct buyer contact and you want the possibility of repeat business, you should consider Reddit.
You should also try Quora, but it's gonna take a lot more work.