I would fix it up.

You get 3 benefits:

1. Continued income from it (people aren't going to continue to buy it if the bad reviews keep coming in).

2. You keep/improve your reputation. If your software isn't working well for any reason at all, it can give a bad reputation to the person/company attached.

3. You increase the chances of a good buy-out in the future, over what it would currently bring now.

We all know it can be difficult to get traction on a project, not to mention paying users. You already have proved this one can get both.