Can the seller retake the car back?

Do you have a contract? Do you have receipts of payments? Who has the title? I'm afraid so. Unless you have the title, a contract & receipts.

if he transfered the ownership to you , well its  yours , he is breaking the law if he sold you the car and didnt fill out the transfer papers , its yours legally , you own it .

Clarify this in your mind.  "I intend to buy a used car from someone, meaning I intend to pay him the full amount".  UNTIL THAT HAPPENS he owns "his car". He is waiting until YOU COUGH UP THE CASH.

I personally would not give you the key to the car or anything.  It is mine until I see the last DIME in MY HANDS.

Every store that sells goods works on the same principle. Car is no different.  You can't register without the title, and he be a fool to give you the title without FULL PAYMENT.

IF IN THE MEANTIME WHILE YOU ARE TRYING TO SCORE $400.  someone pulls up and pays him the full price he is asking...he gets the car and you can pound dirt.

First to pay is the first to get the prize.

In order to register a car you have to have the title.  Assuming that's been done he could still repo the car for non payment.

No, not if you have the title, free  and clear.

Is It?  Is  His name Anywhere on it? If so, Yes.

Also, if it goes to court, if he has written contract, judge Might give it to him. Might not, also. If I were judge, I'd not let him take it But, I'd order you f to make final payment in reasonable time. And both of you Out of my courtroom, if I see you again. it's contempt.

If the title is in your name, it is legally yours.  If anyone takes it, report it as a theft.

You're both stupid. If the title is in your name and he has no lien on it, the car is yours.

First of all only an idiot would give up their title without being paid in full. It's either that or you got the car from a caring family member. Either way it would take  a good year in the court system for them to repo your car. If it's a family member go take a loan out against the car for the $400 and end the drama. They did you a favor.

No.   You have a sales contract,  he accepted the money and you got the car and it is registered in your name.   The sale is over and he has no claim on it UNLESS you do not make payments,   then he can take the car on a repossession as the lender,  not as the former car owner.   

Not enough details. A judge could order the car given to him.

You have not stated your location or details of the contract.

Most US States have titles and contracts of sale.

Generally yes if you owe the seller money.