Not a lawyer, but: In many countries, a purchased item is your property once you removed it from the premises of the seller. In practice, this rarely makes a difference. You have entered a contract with the seller where the seller has to deliver the product, and you have to pay the money, you did your part, they have to do their part. There would be a difference if the item was stolen while in the store, or damaged by fire, or if the store went bankrupt and bailiffs took the item. If these rules apply in your country, then what they did is not theft, otherwise it would be theft (in all countries, if the store removed the door from your home after it is installed, that would be theft).
You paid for a door, the store owes you a door. You have a legal contract. Both sides are bound by that legal contract. They have to do what the contract says (delivering the door that was displayed in the store), if they can't, then they have to do the nearest thing that isn't to your disadvantage, like delivering a new door. Or possible a different door that was on display. They can't just declare your contract invalid because it suits them better.
That wouldn't be the case if this would put the store at an unacceptable disadvantage. For example, if thieves had broken into the store and stolen ten doors, including yours, the store might get away with returning your money. Since they intentionally sold your door again to someone else, I don't think they could use this as an excuse.
I'd go once more to the store and ask them whether they want to deliver a door to you, according to your contract with the store, or if they want you to get a lawyer. A letter from a lawyer might work wonders. (Or of course the lawyer might tell you that I'm completely wrong, but they don't know that, so telling them that you will hire a lawyer might be enough).