We have a problem of damp in our kitchen.
The source is from the upstairs neighbours.
They have been uncooperative and have failed to do anything to fix the problem for 5 months.
A real estate agent told informed us that the neighbor directly above us is planning to sell.
Are we allowed to tell the real estate agent that there is a problem in order to protect the potential buyer.
It would be permitted (and even obligatory) to warn potential buyers about the problem, and to save them from loss (this would be included in the halachah of hashavas aveidah, which includes saving people from loss). Furthermore, you could warn the people upstairs that they should fix the problem, or else you’ll tell potential buyers about it.