Let me make this too simple is Trump wanted to steal documents for some nefarious purpose such as passing them to a foreign government he could have easily photo copied them while he was president and passed the copies along to anyone, so let us not try to make something out of nothing.
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.