If you regret your decision because you feel like you got ripped off, gather documentation that shows in what way you were wronged. Request a meeting with the dealership manager and calmly present your case. While it's unlikely you'll be met with much consideration if you are the one who signed the original deal, this might work if you are acting on behalf of someone else, such as your college-aged child buying his first car or an aging parent. If meeting with the dealership gets you nowhere and you feel you have a legitimate case of price gouging or fraud, contact the attorney general's office in your state to discuss your options.
If the car buyer's remorse is the result of car payments that are too high, you'll have a harder time making the case for the car's return, since you should have researched this thoroughly before signing the contract. Still, you may be able to work something out with the dealership to get you out of this car and into a less expensive model, especially if your financial circumstances unexpectedly changed shortly after your purchase. Start by talking to the salesman who sold you the car first, but expect that he'll need to involve the sales manager and possibly even the dealership's general manager for a new deal.
If the reason for your remorse is a problem with the car, then gather all the documentation that shows the mechanical problems you have experienced. This will likely require multiple visits to the dealer's service department. Make sure your complaints are noted in detail each time on the repair order when you bring in the car.
If you feel you have a lemon car because the problem hasn't successfully been fixed or because new issues keep cropping up, research the lemon laws in your state and bring all the documentation to the dealership manager, asking him to replace the car. If that doesn't work, look at making a lemon law claim through your state.
Finally, if you have car buyer's remorse simply because you don't like the car, you'll probably have the hardest time getting a dealer to work with you. Certainly give it a try if there is another model that this dealership sells that you'd really like, especially if it's more expensive than the car you bought. If that approach fails, you can look into selling your current car privately, which will net you more money than if you traded it, and then buy what you really want. But, remember that you may be upside-down in your car loan, requiring an additional investment on your end to get that loan paid in full.
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