There are two rent numbers that are often quoted, the "Pro-forma" and the "Actual". Pro-forma is the amount the property could generate if all units were rented at market rents. The Actuals are obviously the amount actually received by the owner .
If the owner is occupying one of the units that should be noted and a pro-forma number given. Either way that you do it you should be able to give the actuals to the interested buyers when you enter into contract. The buyer will want to see the actual expenses and incomes for the property.