Carfax doesn't really tell you anything, I bought a car that had a whole new door on it, but Carfax didn't tell me. Only if the dealer voluntarily reports it does it get in Carfax so if the dealer relies heavily on it, be suspicious, but with a new car, shouldn't have to worry too much. biggest scam is financing through the dealer and extended warranty. Get the loan through your bank, not the dealer. Also, the extended warranty excludes everything that is most likely to go wrong with the car in the first few years anyway. If it doesn't go wrong in the initial warranty period, covered by the manufactures warranty, not likely to go wrong, like 99% not likely.
as far as financing, the dealer won't give you the best rates, your bank will and your bank is more likely to work with you if you hit trouble.
lastly, if the dealer advises you to do something shady, leave! If they advise you to be shady, they are shady.
oh, another scam is to cheat you on your trade-in. Take the trade-in to several dealers, pretending you'd trade it in, and see what they offer on an equivalent value car. I've done this, saying I'd was looking at a car around $23,000, and my trade in was valued anywhere from $800 to $4000. but you need to be sure you do this on an equivalent value car.
one other trick is the bait and switch. i talked to a dealer, they gave me a price, then when i agree to it, then they came back and said $1200 in destination charges. That's just another way to stick you. Taxes, license, title, those are legit extra charges. Destination is NOT. Another dealer pretended to rework the numbers. first offered to sell me something for $17,000 and give me $900 trade in. I said no. Then he "talks" to his manager, comes back and says, ok, I can give you a better deal. $15,500 and $700 for the trade in. hello. I walked out and they called me for 6 weeks with a lower price each week on the car I was going to buy. Bottom line, just be prepared before you go. Know what you can afford. Know that your bank will give you a loan. and do NOT buy the extended warranty or pay destination charges. We all want that new car now, especially when it's so close. But if you tell them no and leave, they WILL call you back if it's something like destination charges.