The existence of a 72 hour cool off period must be some persistent urban legend that won't die. There is no such law in Oregon, probably anywhere.
Talk to the dealership, ask for your money back or to apply it toward a different car. A reputable dealership will work with you. If you were shopping at a disreputable one, then now you know why it's disreputable. Look to your warranty (if you got one) or sales agreement which may provide you with remedies. Otherwise, your best bet is an uphill battle based on a claim for fraud or unfair trade practices. You might want to consult with an attorney about bringing these claims, but unless its a particularly valuable used car, you're likely to end up representing yourself in small claims court. Good luck.
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Oregon's lemon law applies only to new cars, and as my colleague pointed out, a 72 hour contract rescission period isn't applicable in this type of a situation.
Depending on the facts regarding what you were told when you purchased the car, however, you may have a viable trade practices violation case.
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