I was actually going to PM you and see if you wanted me to send you one of my cars for the achievement of a car from every manufacturer or w.e but your recent game history is hidden and I wouldn't want to just give credits away.
I think it would be better if you don't hide your achievements and then people can actually see if you need that achievement or you are just wanting to scam credits. And what achievement do you need for somebody to buy a car from you for a lot of credits?
I am always wary of people who hide their achievements, makes me think they have something to hide
You can unhide them through the Account Settings option on your Xbox 360 console's dashboard as far as I am aware. You don't really need to modify cars to compete in races, as the AI use stock cars. Half the time, you'll get a suitable car by levelling up.
It's only some of the more obscure races in the Event List you'll need to buy anything for, and even then, you'll earn enough by playing the other races. There is no achievement for selling a car for a lot of money. If you want totdo it for the minimum possible
loss, take a VW Rabbit (5,000 CR), and sell it on the Auction House for two hours at 1,000 CR. Someone might buy it. Maybe.