If it was in the US, the statute of limitations for being prosecuted has probably long passed, so they wouldn't have to worry about legal trouble (not sure about international laws). But as the items are stolen, they ALWAYS remain the property of who they were stolen from (in this case, the studio). If they make a 'public appearance', or are ever found somewhere (like an abandoned storage unit or something), the studio can always re-claim them and who ever has them would have to give them back. Even if they bought them from someone not knowing they were stolen.
The only way it would be safe to publicly display or sell any of it would be if the studio says they won't come after the items if they ever turn up.
Paramount did this a number of years back when a huge collection of Star Trek props were auctioned off. Many of which, rumor had it, had 'walked out the back door' so to speak... Paramount basically said 'If it's already out there, we won't come after it...'
Otherwise that seller might have lost most of their collection to being confiscated...
But other studios have done this. Fox came after a drawing Jack made of Rose from Titanic when it came up for auction a few years ago, Lucasfilm confiscated an R2-D2 head that was supposedly 'rescued from the trash' I think from the prequel films, and WB is known for going after ANYTHING Potter related if they didn't specifically sell it or donate it for a charity auction or something... (They may have calmed down a bit by now, but if a really key, hero prop suddenly came up on an auction site, I bet they'd still go after it...)