Take a poo on the door step.
No, it is a gallery, one should defecate in the middle of the floorspace.
My advice would be to write an email to him demanding an apology and outlining that you have been a regular customer. Writing is so much better than talking in person because you get to construct your argument in a calm, calculated fashion, without the other person butting in or walking away.
Tell him that if he doesn't reply you'll happily inform everyone you meet of his disgusting attitude.
No, it is a gallery, one should defecate in the middle of the floorspace.
In these situations I wish I was a billionaire and buy his little shop, and him, out
Art / gallery owners are usually pretentious snobby arses.
Get your other half to go in there in a few weeks, make some enquiries, go back another week later and buy the painting for you. Take it back yourself, go inside and ask him to come outside with you. Proceed to burn the painting in front of him whilst simultaneously giving him the finger.
That'll learn him.