Well, let's start with everything in a production either:
1) Belongs to the studio
2) Is a rental
If it belongs to the studio and it "walks off the lot", it is stolen, PERIOD. If it is a rental, then the renter owns it and can sell it. This is the case of my hero Starbuck FN 5-7 pistol. I got it from the armorer who had rented them to the production.
Collectors want to dance around the issue that they may own stolen property. But if the studio doesn't official release it, it is stolen property, whether the prop master takes it, or an actor or whoever takes it. Unless the item was given to them or they received authorization to take the items. That is where the gray area is. Did they get approval? By someone with the authority to give it? Yeah, it is not an easy question.
That being said,the studios will not go after people who have props, because they generally don't value the stuff. So you don't have to worry (other than Lucasfilm) about studios coming after you. Plus, most props aren't expensive enough to warrant legal action (But some ar. Remember the FBI busting that guy for the stolen Spiderman suit?).
The Christie's Star Trek auction woke SOME studio people up. TV Studios are waking up a bit. NBC/Universal gets it and they understand the value of these assets since we made millions for them in the Battlestar Galactica auctions. Disney/ABC does because they have an awesome guy in charge of assets. CBS does and is trying to get that machine going.
As far as movie studios, well, there are lots of issues with them, but generally our efforts and those of others are starting to gain traction and making it easier for them to understand the issue. This is the key problem. Studios have NEVER valued this stuff, and now it is big business. So now they realize they have to have serious asset management in place to deal with these assets.
So the industry is really changing and I think that is good for collectors. Less uncertainty about authenticity, more availability of items, and an above the board hobby, rather than an underground one.
Alec