Granted there's always some may have been stolen on order [1], but for the rest, they would be more or less unsellable.
[1] - https://en.wikipedia.org/wiki/Isabella_Stewart_Gardner_Museu...
Which rather defeats the point I'd imagine, for most thieves.
(When one of Edvard Munch's 'The Scream' paintings was last stolen, it was to force the Norwegian police to divert resources from the investigation of an armed robbery which had been conducted a few months earlier.)
I can also see it being a good bargaining chip to have should you find yourself under prosecution.
"Oh, it so happens I may be able, through my contacts, to influence whoever stole this priceless work of art to return it to the owners, where it rightfully belongs... Well, at least if I am somewhat motivated, so, what's in it for me?"
I have read that stolen masterpieces are sometimes used as currency for (high level) criminals. Similar to how Tide bottles were (still are?) used as a value exchange within homeless communities, “I owe you $X00,000 but would you accept this Picasso instead?” passes some stolen art around at a large discount to “market value”.
In many cases there's also a reward for any information regarding the stolen artwork. So that makes it even harder to resell those artworks.
The world is a big place and the rule of law we trust to just work in the West is certainly not the norm everywhere.