One way to get what you want is to persuade prospective buyers make this be a condition of the sale. Whether or not this works would depend on getting the ultimate buyer to care about this wall, when they may not be aware that there is a problem. If you limit yourself to truthful non-libelous statements, you could post a sign on your property and hope that potential buyers might see the sign.
It is possible that the wall is in violation of state or local building codes, for instance if the wall is over 4 ft tall, it requires a permit. If the wall was not permitted (where a permit is required), that does not mean that the city will require the wall to be removed – municipalities do not generally take sides in such squabbles, and you would have to relate your complaint to a compelling interest of the municipality, e.g. is there a palpable threat posed by the wall.
The wall being on the property line may be in violation of setback requirements. If it is actually over the line, you have a somewhat different problem, that the neighbor could now be in adverse possession of your land (depending on how long it has been up). A preliminary step in that regard would be to get a survey, since homeowners don't generally have authoritative knowledge of where their property legally is.
If you have actually been harmed by the decaying wall (it does not seem that you have but you can think about that), you might have a basis for suing the neighbor. Even if your suit would not ultimately prevail in court, involving your attorney could be sufficient motivation for him to remove the wall.
