Servicing a marker is much like servicing anything else. It gets taken apart, cleaned, visually checked, any worn seals, o-rings, etc., are replaced, lubricated, re assembled and the settings checked and adjusted as required.
The cost depends on where/who services it and what parts are needed, so are variable.
Sellers that have just had a marker serviced by LiPS, Eclipse, Dye, or any other recognised establishment are instilling more confidence n their goods for buyers (i.e. there shouldn't be anything wrong with it if somebody confident has just serviced it).
Bit like buying a car really, do you buy one with a recent service & MOT, or one that has no service history?