1. Everyone can sell on here, it just depends if you are lucky enough that buyers find you and want to spend money on you.
2. You just price them on your own because as far as I know there is no perfect way to do it. A good idea though is to try to put a price in a mid range. Not too low and not too high to avoid either undercutting the market or scaring off the buyers.
3. It is a bad idea to use someone Else's models. Best if you make your own from scratch. Please try to make something unique and innovative because if you are a beginner and you are copying something like a car there will already be like 40 or more versions of the same car done better so no one will ever see it pop up in the search engines.