assuming that list_type is the name of the select box to choose from (values : selling or rent)
use it this way
"sale_price" => "required_if:list_type,==,selling"
what does this mean?
:
the sale price is only required if the value of list_type is equal to selling
do the same for rent_price
edit
public function rules()
{
return [
'list_type' => 'required',
'sale_price' => 'required_if:list_type,==,For Sale',
'rent_price' => 'required_if:list_type,==,For Rent'
}