I'd replace them, they're definitely broken. Part#51321938884 getbmwparts has them for $2.50ea and they're genuine BMW parts. Pelicanparts.com sells the same ones for $4.75ea or the generic ones for $2.50 and ebay has generic ones for like 6 bucks each. Your best bet is to get 4 Genuine BMW sliders from getbmwparts.com. It's only 10 bucks, i'd replace all four of them.
This is what they're supposed to look like. Make sure you clean the old sticky grease out of the tracks and replace it with fresh grease.