The thieves may have had buyers lined up before they stole it.
Even so, because the buyers have to sit on the stolen property pretty much forever, until the record of the original owners is lost through time, there is no way the buyers are buying it for any ability to profit from the sale.
That will affect the value somebody would pay for it.
Based on the evidence at the museum, it is doubtful they were experinced ART thieves. This does not mean they were not experienced theives. They cut the paintings out of their frames according to an article I read yesterday. This would dramatically lower their value. An art thief would not to do this. However, they did remove the tape from the video recorders, but it was already on the computer hard drive. They also were dressed as Boston policemen.
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.