Porsche 911 - Accepted deposit but sold to another. - King Onid
I initially accepted a deposit of £100 from a dealer for my Porsche and signed a purchase invoice for an agreed amount, but the next day I received a higher offer from a private buyer and accepted it, selling the car to him.

I informed the dealer as soon as I sold my car and returned their deposit, but they are now threatening legal action because they claim I violated the contract.

I'm aware that I made a mistake, but I had the deposit less than a day. Am I not able to change my mind when selling a car?
Porsche 911 - Accepted deposit but sold to another. - bathtub tom
I initially accepted a deposit of £100 from a dealer for my Porsche and signed a purchase invoice for an agreed amount,

I think you're in serious doo-doo. The dealer claims you had a contract and I agree. Get a solicitor!

Porsche 911 - Accepted deposit but sold to another. - Andrew-T

When a deposit has been paid, a buyer may withdraw from the agreement, perhaps forfeiting his deposit. By accepting a deposit the seller has agreed the terms of the sale and can be expected to complete it.

In effect your buyer (the trader) has been gazumped and is rightly indignant, especially as you have a written agreement. You can only wait and see what happens. Of course there have been cases where the boot has been on the other foot .... I remember buying a car from a dealer about 40 years ago, who soon came back to me saying a mistake had been made and I should have paid more. I agreed to reverse the sale and never set foot there again.

Porsche 911 - Accepted deposit but sold to another. - Brit_in_Germany

Maybe the dealer is just bluffing. I am sure that if it was the other way round and you put a deposit on one of his cars which he then sold to someone else, he would just say 'tough'.

Porsche 911 - Accepted deposit but sold to another. - daveyjp
Unless the contract has a specific performance clause you are quite within your rights to pull out and return the deposit.
Porsche 911 - Accepted deposit but sold to another. - Andrew-T
I'm aware that I made a mistake, but I had the deposit less than a day. Am I not able to change my mind when selling a car?

What would you have said if the positions were reversed ?