You are making this both too easy and too difficult, even though those seem to contradict each other.
It isn't that easy to "take a picture with a phone" of the classified documents, since phones, cameras, and other electronic devices are generally not allowed in SCIFs, where the documents are kept -- and the exceptions are for secure devices (such as computers, copiers, etc, that are needed for the work being done). Your suggestion of just copying or taking a picture is not that easy -- the controls on those types of items in a SCIF tend to be extreme.
At the same time, if the President wants to share Classified information it is quite easy -- he simply shows the actual classified documents with whoever he wants, such as documents President Trump was
reported to have shared with Russian officials visiting the White House. If a President wants to "sell" Classified documents to anyone, there isn't anyone that can really stop him (other than Congress impeaching the President, at which point he could be tried legally) -- but that prosecution can only be done after the fact, after the President has been impeached for already having sold (or tried to sell) information.
OTOH, a former President should have a difficult time sharing Classified documents as he should no longer have any -- or even unclassified documents from his Presidency (other than copies supplied by the National Archive). Granted, that wouldn't stop a former President from "selling," or just sharing, classified information that he can recall.