The Mayen M6 makes a low profit when you buy it three star and rebuild it. At least the Mayen M6 is a fairly easy car to rebuild. There are much better cars that can be purchased for the same price, or lower, that provide a much more impressive profit margin.
Purchase Price: $20,718
Repair Cost: $10,630 (+/-)
Sale Price: $45,192
Profit: $ 13,844 (+/-)
As you can see, the Mayen M6 makes for a rather poor car to rebuild in Car Mechanic Simulator 2015.
Rebuilding vehicles for profit in Car Mechanic Simulator 2015 is by far the most profitable activity in the game, assuming that you're picking the right vehicles. Of course a solid supply of spare parts helps too. Here are a few links to additional information that can help you get started in the Car Rebuilding side of the game. This will help you improve your overall game experience:
Rebuilding Guide with Cost vs Profit for all Vehicles in Car Mechanic Simulator 2015
The Auctions Center - The place you buy vehicles
Complete Series of Vehicle Rebuilds for Car Mechanic Simulator 2015
Tunerz Paradize - The place to get special parts to trick out your rides