If something is sold, it means that someone gave it to another person in exchange for money.
When something is sold, it means that someone wanted it and paid for it. It's a way to get things you want.
Examples:
He sold his old car.
She sold her bike to buy a new one.
The store sold all its toys.
I sold my sandwich to my friend for a dollar.
I sold a million toys in one day!
The man sold his house and moved away.