i guess they get the same salary than other retail. No luxury salary for luxurous shop.
I have a girlfriend who did an intership at Chanel compagny in New York. She helped to prepare fashion shows. So she met important persons at Chanel; they said that they were happy with her work and at the end of the internship they offered her ... 3 lipsticks and 1 parfume.
So i guess they are not more generous with sellers in the boutiques