Your best off making it sellable not "look sellable". I'm just giving you a hard time, but honestly I wouldn't conceal an issue.
Your basic steps are going to look like this, specifics will vary based on your situation.
1) Clean and kill the mold. This will probably take some wiping down, plus a chemical treatment. I have had good luck using wet n forget outside and I believe they now make an inside formula. Honestly lysol would probably be enough.
2) You will have to prep the ceiling for paint. This is going to entail removing the paint blisters, filling the voids left by them and then sanding the filler flat.
3) You are going to need to repaint the ceiling. If the mold has stained the ceiling I would recommend a starting with a stain blocking primer so that is does not bleed through down the road and then at least 2 coats of ceiling paint.
This really should have been step one, but keep the security deposit.