They are works of art and even though WE could not fathom selling those pieces if they were in our possession, they have every right do what they want with them.
Material objects like that are held with higher regard by the people who own them...and much less by the people they get passed onto. I'm not saying his family doesn't respect them or admire them, but they did not make Stan who he was. They are merely a representation of the works he created - not his personality.
We see selling them as sacrilegious, but in the end, they are still just mere material objects that we can't take with us into the next life...