The posting of any image by me representing an actual physical painting is not deemed to constitute an offer, it is an invitation to treat. Any response thereto by you or anyone acting on your behalf in any way constitutes an offer. When that transaction is processed, this consttutes acceptance by me and the contract is then formed when the consideration (ie your money) passes to my acount.
There is a clear and obvious diclaimer at the top of the online shopping page. This states that customers are strongly advised not to make an online purchase without first contacting me to check on availability of the picture.
In the event of money being paid to me in respect of an attempted purchase of a painting which transpires to have already been sold by another party such as an art gallery, my policy is to offer a full refund minus any administrative or other charges levied by Paypal, or the banks.credit card companies.