hi,
These are probably sterling silver beads, or silver-color beads with antiquing.
Indonesian and other Bali-style beads are known for their detailed work. The antiquing helps the detailed work stand out. The recessed areas are antiqued, and the high points are generally shinier.
Sterling silver also slowly oxidizes on its own, without help from antiquing solutions. If the beads have been sitting around for a while, this could be what is rubbing off on your hands.
It can also rub off on your neck, wrist, and on your clothes
To help prevent it rubbing off onto customers' necks, white blouses, etc., you might want to use a polishing cloth to hand-polish the beads when you're done stringing them.
I do not recommend dipping the beads in a liquid silver-cleaning product, because that will change the look of the beads. It will strip the antiquing out of the recessed areas, as well as brighten the finish of the high points.
--Polly