Do you clean and lubricate the sunroof tracks regularly? If no, this may be all that needs to be done.
I use the BMW RHF 1 sun roof lubricant, because it is available in small tubes, unlike the Mercedes sun roof lubricant, which is only available in a tub, which typically costs about $125. The BMW sun roof lubricant can be picked up for <$15 for a tube, which has more than enough lube for 3-4 generous applications.