Ask HN: How do I sell the potential of my app?

Hi everyone, I set my latest web app live last month, it has been received fairly well so far considering I haven't done any marketing or mentioned it much at all.

A few days ago I received an email from a development company asking if I'd be willing to sell my app to them.

Now, I hadn't thought about this up until now. As it's a new app all I've got to go on is its potential - I have no idea how much this thing can make per month a year from now. All I know is how much better it could be if I had the time to develop it further.

If I was to try and sell it to this company, what would be the best way to show my app's potential?