bad idea indeed
net profit margin you are looking at max 10,000 to 15,000 rs each car , these cars are not taken as investment cars. Since you can get a MEHRAN CNG for cheaper as this shape has been there since 1989 so there is not much you can make on these cars no matter how good you think they are.
If you want a good return from mehran in short time then buy atleast 6 to 7 atleast and sell them your self not through a dealer that way you will make good money
selling two mehrans is umm not something that id call wise or something as investment purpose