I think you'll probably lose it. The seller may have accepted the deposit in good faith and then turned down other offers for the car or incurred some costs . There is no harm in asking though.
It may depend on how you bought it. Was it online, over the phone or did you see it? Is it a private sale or from a showroom?
If a Side of the road Purchase I doubt it, has the car got any mechanical problems? If so, state it to the seller, what paper work? was it verbal? will not stand in a court unless said in front of a witness. why have you had 2nd thoughts?
I went ahead and bought it. He said it needs to have a new cam belt shortly, but looking that up on the net that sounds right to renew it after so many miles