It's recommended that it's rebuilt EVERY 100 hrs...cost is $300+
Ask the owner if it still has the ceramic washers in it, these have a tendency to fall apart and ruin the whole engine. They were replace by metal washers, I'm not sure when that happened. If still ceramic I wouldn't run the ski until replaced. Have him prove it with a receipt if he said he replaced them.
And as for the washers I was told by the dealer I get it serviced at that mine has upgraded/ changed washers because they had problems for awhile with those original ceramic ones blowing apart if that's what your referring to but I have not paid to have them changed and have had no problems with anything on the ski... But the ski is all stock and like I said just done services and routine maintenance that have been done at only a sea doo dealer. I dont know if that answers your question but I can try and find out more about it if you want.*