If I win an auction on ebay , lets a car and the last price a bit was really low do they have to sell it to me or can they just change their mind ?
yes unless they have a reserve on it that was not met when time expires it yours
It's yours!
Be glad you got a great deal!

Once a buyer puts an Item up for auction they cannot back out.
It's part of the contract you consent to when you put an item up for auction.
{The contract swings both ways though, if you win, you have to pay for it.}
But as long as you were the highest bidder, what ever the item was is yours.
Beware though, sometimes if the item is really valuable it can "Get Lost in the Mail". (Things can get lost in the mail but it RARELY happens) So if you pay for an Item and it never arrives, makes sure to contact Ebay immediately and report the user.
The easiest way to tell if the user is safe is by their feedback.

Hope this helps!
They should sell the vehicle to you so long as you met the rules of the auction (no reserve specified, you me the buyer quality guidelines they set) and submit your payment on time.

Are they required to sell it to you? No. They can elect not to but you would be able to post negative feedback that they reneged on the terms of the auction.
Yes, unless there was a reserve price. You need to try to resolve the problem though email. If unsuccessful after many attempts leave negative feedback, and follow up feedback. Check out the link, completing the sale.
NOT UNLESS THEY HAVE A RESERVE OR BIN (BUY IT NOW) THEY HAVE TO SELL WHATEVER PRICE IT ENDS. IF THEY SAY IT WAS BROKEN, TELL THEM TO SEND YOU THE BROKEN PARTS AS IT IS YOURS. SUE THEM IF YOU HAVE TO. LOOK, IF IT WAS A CAR THAT YOU OVERPAID BY SEVERAL THOUSAND DOLLARS I BET THEY WOULD WANT THEIR MONEY. THEY LOST, YOU WON. SEND THE MONEY AND GET YOUR ITEM. GET EBAY INVOLVED AND PAYPAL IF YOU MUST.