[Calvin Locke]

Back in the day... 80-120 years ago, smarter thieves would pre-sell the work a handful of times, and have copies made in advance.

Skeptical buyer? No problem. Bribe a guard to let you in after hours, position a copy in back of the original, invite the buyer in to mark the back of the canvas, steal the painting, ship off the copies as the news of the theft works it way around the globe, impressing on the buyers that they each have the original.

What are the conspirators going to do if they ever found out about the forgeries?

There's a somewhat recent book "The Rescue Artist: ....", on the Winter Olympics Munch theft. To fill out the book, stories of other art thefts are woven into the account.

[Daffynition]

Thanks for prodding my memory.

After your mention, I believe I read a review of that book, but never pursued reading it. Sounds Dan Brownish.