I wanna open aa supesupercar showroom any tips?

No it won't work. Markups in dealerships are nowhere near 100%, more like 10% which means that if you buy a car from a manufacturer that costs $250K, the dealership price will be somewhere in the vicinity of $275K. Your competition is going to sell that car at the $275K mark (less negotiations) so why would anyone buy the exact same car from you for $500K. And remember, the dealership has overhead to pay out of that markup, so your profit will be less. Dealerships make their money through selling add-ons (i.e.,maintenance warranties, GAP insurance, etc.) and through the repair shop.

Then there is the problem of getting that showroom built and the dealership deal. Most car manufacturers limit the number of dealerships in an area to limit competition - the more expensive the car, the more limited the dealerships. And then, you'll need a few million to build the actual dealership - the manufacturer, before he gives you the license to open the dealership, will want a brand new, expensive looking facility and they are not cheap. An article in the San Fran Chronicle indicated that the average dealership costs about $11 million - since that includes all brands, you can expect that a dealership selling top end cars will cost more. This all has to be self-financed and in a best case scenario, a bank will end only about 25% of the cost - meaning you need to come up with about $3 million on your own.

And this all assumes that Ferrari or Lambo will even allow you to open a dealership - more likely until you actually get experience in the auto retail industry (salesman then manager, etc.), they will not grant you the franchise which means you cannot buy cars as they roll off the line and selling these things used is even harder and less prosperous (the people that can afford these want them new and usually do not trade them in, never mind sell them to a used car lot).

Start with a bunch of supercars, find space, open showroom.