Perversely, it was probably putting the floor membrane in that made the situation worse.
Damp control using barriers and membranes isn't quite the easy solution that people tend to think it is. In simple terms, moisture given a barrier in one direction will always try to find another, easy, route. In this case... up yer walls ;o(
The "French Drain" is classic. A big trench full of clean stone to "soak up" surrounding moisture. Possibly a little drastic for a mere porch, but would certainly help.
Damp is squeezing up from under the floor membrane and climbing the walls. For a barrier or "tanking" system, the floor membrane should be linked to a tanking membrane on the wall, especially where it's lower than the house floor level. (Jargon alert: "Sub-terranean")
So many ways to go: Either...
Knock off all internal porch plaster ... tank with bitumen or, better still, a cement-based waterproofer such as "Vandex". Then re-plaster.
Or, after removing plaster, NO tanking... just replaster with lime plaster. (Completely breathable, so moisture just evaporates through it.
I won't go on. It's all getting too complicated. It's easy to be too purist about this. In your case, you just want to sell. I would just remove the suspect plaster and replace it with a simple old-fashioned lime mix. Then paint it with a porous NON-vinyl emulsion. Builder's Merchants stock basic "Trade" or "Contract" emulsions for new plaster (porous).