Usually, no extra contract is needed to keep copyright. In most countries, selling a print transfers ownership of that physical object only, not the copyright in the image. That means the buyer generally may own and later resell the print itself, but cannot legally make copies, distribute reproductions, or sell new prints without the photographer’s permission unless copyright was explicitly assigned.

So the key point is:

• the buyer can usually resell the original print they own
• the photographer normally keeps reproduction and commercial rights automatically

Trying to forbid resale of the physical print is uncommon and may not be enforceable in some places. But preventing unauthorized reproduction is already covered by copyright law, so a special boilerplate document is typically unnecessary unless you are granting or transferring rights beyond normal personal ownership.

If you want certainty for a particular country or for high-value work, consult a local lawyer, but for ordinary print sales the photographer generally retains copyright by default.