Good question Jaime.
Welcome to the forum, and thanks for joining us.
Now, to try to help with your question...
I read the IRS page on the topic: http://www.irs.gov/businesses/article/0 ... 41,00.html
I also skimmed through most of the links on that page.
From what I can tell, the credit is only available for the original purchase of the vehicle from the manufacturer. I see NO language about how long the party must keep the car. It does say that you cannot purchase the car with intent to resell. If I were to venture a guess, I would say that you could sell the car within a few weeks of buying it. If you sold it the following day, I would guess they would claim that you bought it with intent to resell. But if you kept it for a couple weeks, they surely couldn't force that issue with you.
But, know that the existence of the tax credit will make it hard to sell the car used right away. The person buying it from you cannot get the credit (based on my understanding that it must be purchased from the manufacturer directly). So, you would have to price the used vehicle at a fair discount from msrp because it's used and not new, and you would have to deduct the amount of the credit that the buyer would get if they bought it brand new.
So, I hope my opinions are helpful to you.. but know that they are only opinions based on what I read on the site. I can't say I'm positive on the topic.