well I see no issues.
My first Crown was Sindh registered (sold within Lahore).
My Altis was Sindh registered (a friend from Peshawar bought it).
(Both were daily driven vehicles in Lahore with original number plates).
The only backdraw of running Sindh registered vehicles in Punjab is the slightly less price of vehicle.
Suppose if Punjab registered vehicle price is 5 mil in Lahore, then same vehicle but registered in Karachi would be 50K less. (Same happens with Punjab registered Vehicles in Sindh) 
Means you buy it like slightly cheap and you will sell it slightly cheap again. So the equation is balanced. But make sure that it has original number plates and file is perfect too.
There are lot of agents in Lahore who are giving services to transfer the ownership of Sindh registered vehicles. Its easy.