Can you back out once you place a deposit ....guidance?

I am an attorney, and the answer to the OP's question is it depends on what if anything he signed or agreed to (email generally will count) and state law. I would advise the OP to be honest with the dealer he has the deposit with, and ask them to make a price concession or he will purchase from the other dealer. This, of course, depends on the amount of deposit and if he is willing to loose it all, which is the probable outcome.