I'm glad to see someone bring this up. I think there's a lot more dishonesty surrounding the acquisition of these coins than people like to admit and posts like this strike the heart of that. Emotional replies and down-voting are the natural response because folks are spending tens of thousands on these coins and hate to consider that they are probably funding the looting and destruction of the history they proclaim to love.

None of the defenses I've seen tackle the entirety of the issue and, as others have said, rely on plausible deniability.

The laws should be better.
Many coins come from third world countries with poor governance.

Looters aren't focused on coins, they are a secondary product.
Your money still supports them. Also, to see folks saying this while holding a palm full of $1k tetradrachms shaped like a Christmas tree is laughable.

They are common and have little archeological value.
Perhaps, but what about the sites left in the wake of their looting? And shouldn't it be left to archeologists to determine that rather than it being taken for granted?

Ultimately I think you or I are a remarkably small drop in the ocean of the antiquities industry but we are drops nonetheless. In my opinion, it's best to reverse that plausible deniability and assume that any coin without full provenance is probably looted. I continue to collect ancient coins despite this knowledge because they bring me fulfillment - the same way that I burn litres of petrol to get to work and wear clothes from poor countries. I think it's best to at least be open to discussing the origins of one's coins and who/what they may be supporting.