I went through one case where someone I thought I knew well was accused of and convicted of a crime, in his case child pornography. It is quite difficult to deal with such cases, as you know quite a few details on one end of the story and practically nothing on the other end. Indeed, the trick is not to rush to judgement (I was appalled, not only by what was found on his computer, but also by the sheer stupidity of how he advertised his behaviour with the children and how no one, including myself, had noticed), but even so, what do you do during due process? Do you support him, because he's a friend or drop him, because he's a criminal or drop him until he's out of prison and paid for what he did? Not easy.
Without pre-judging the present case, there are some easy points to make. Of course, there are plenty of dealers who buy treasure coins with a shady background. It is correct that the law is an ass in many countries. It is far too easy to say such dealers are morally wrong, just as it is too easy to say that they can do whatever pleases them. I would argue that there are two variables, I'll call them due diligence and heritage.
Due diligence is not being an obvious law-breaker. If someone comes into a shop with over 100 4th century small Roman coppers and claims they're his uncle's collection, it should not be necessary for a dealer to check import and export documents and alert the police in two countries if they (or the uncle) can't be produced. If the same person comes in with the same coins and says they come from an illegal dig, the dealer should refuse to buy. That leaves a whole grey area where a dealer can suspect, but doesn't know if something is fishy. Maybe the dealer, rather than the state should decide what to do in such cases.
Heritage refers to the importance of some coins for national history. That has nothing to do with its value. A Roman coin found in the Baltics is important, as few Romans are found there, so they have a story to tell on trade in ancient times. That same coin found in Crete is yawningly common. A dealer who is confronted with a coin he recognises as stolen from a national collection or otherwise of special value for a country has the duty not only to refuse the deal, but to help the police trap the robber.
Putting the two together, you come up with the question, "what does the buyer know about the coin?" Even so, nothing is ever simple. If I would buy a duit from a dealer in Amsterdam today, chances are that it was found recently by a construction worker employed for the construction of the new metro line. The coin is extremely common, has no "heritage" value and if the find were treated according to the law, a team of archeologists would have stopped the work, causing further cost overruns and achieving nothing. I can't even tell if the coin came from the metro works or from a collection. But what if it is a gold coin struck in Batavia? A very unusual find and coin, but probably not from the construction works. What if the dealer tells me it was found in Amsterdam recently? At what point should I start asking questions?
Maybe the case against Salem Alshdaifat will provide some clues?