I went to a car dealership to buy a car, I deposited 300 dollars,he told me he is going to hold the car for 3 weeks,until I bring more money,but is 1 week I can not bring the rest of the money,so iasked for my money ,he said is not refundable,I did not sign any contract,can he keep my 300 dollars,please help.

? I am sorry first you did not sign a contract, that is mistake one.

But of course a deposit to 'hold" anything is normally not refusnded, that is the sellers if you don't buy, why do you thnk they ask for it. If they had to just give it back, there is no need to ask for one.

If you give them money to hold a car for you, and then don't buy the car, they get the money. This is just standard business. You should have asked or found out before you gave them the money