When you buy an original painting, you buy the physical object to have and enjoy. In most circumstances you own only the artwork, not the copyright to it. That remains with the artist unless they specifically sign over their copyright to the buyer, or it was done as work for hire, or the copyright has expired.
You don't automatically acquire the right to reproduce a painting as cards, prints, posters, on t-shirts, etc, along with the painting. It's the same as when you buy a book, film, music, vase, carpet, table, etc: you're acquiring the right to own and enjoy the item but not the right to reproduce it.
What can we do as artists to make this clear to buyers? Add a copyright notice to the back of the painting (© Year Name) and include the information in your certificate of authenticity or sale. If you speak to the buyer yourself, see if you can slip it into the conversation.
Go to Full Artist's Copyright FAQ.
Disclaimer: The information given here is based on US copyright law and is given for guidance only; you're advised to consult a copyright lawyer on copyright issues.
