Don't do that RC without test driving that car for at least a full week. If he is interested to sell it he will do that. After that you feel much better. Just keep in mind - that Shelby of yours will have a very stable value by now, that GT-S will drop significantly. Further on - that Shelby is super irrational and emotional Uber-Cool, that GT-S is a modern MB - both cars have a different market perception and image.