Is auction the only way to sell cars?

No. The only ways to earn in-game credits are by racing or selling cars through the auction house.

yeah man every car is available in auction house you cant sell a car without using auction house it can be a pain if your selling multiple!!

I always hated the fact that in FM3, the only way to earn enough money to buy most of the cars you want is to sell cars with crazy paint jobs or tunes for millions of dollars in the auction house. People who just want to race are left out in the cold, and
it sounds like it's the same way in FM4.

you can if you sell it through the storefront. it will be sold locked, you can apply to that kind of car as many times as you want, but can't modify it, apply it to another kind of car, or sell a car in the auction house with the design.

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if it is gifted to an individual you can see everything that went into it, cannibalize it, learn techniques, paste onto everything, sell it, gift it, etc.

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if someones selling a design through the auction house they probably figure if you want it you'll get it that way. asking to make special accommodations to sell you JUST the design they probably assumed unlocked.

Lots of races or sell some unique cars in the auction house.