By law one does not have to have a written agreement for the sale and purchase of items of property. The only contracts that require this are those involving the selling or mortgaging of land and contracts of guarantees.
However, depending on the value of the item in question, it may be advisable to draw up a written agreement to deal with any disputes that may arise.
Yes. If you have accepted a deposit or payment for the item you cannot then sell it to someone else for a higher price and hope to hand back the first buyer’s money. This will be a breach of contract and the buyer may be legally entitled to damages from you.
If you receive a deposit from an interested buyer, and you live up to your side of the bargain but the buyer backs out of the deal, you will generally be entitled to keep the deposit.
You may also have a legal remedy if a buyer backs out of an agreement and this causes you a loss.
If you hand back the purchase money and the buyer accepts it, this may amount to a cancellation of the agreement for sale and purchase.
For selling personal items such as jewellery you do not need proof of ownership.
In general, a contract entered into by a person under the age of 18 cannot be enforced against the child, although the child can enforce it against the other party. However , the contract may be enforceable against the child if the court finds that the contract was fair and reasonable.