It's always advisable if you know little to nothing about jetskis. But, all the dealers I know are booked solid for maintenance, so they may not be able to get to it quickly. You could always call them and ask...I would expect them to charge at least 1 hour labor...probably $80-100??
If nothing else, it needs to be test ridden to ensure it runs properly. "Trailer tests" are not sufficient.
If the buyer won't let you test ride, ask them to take you for a ride or you drive and they sit on the back.
You'll want to make sure it runs smooth, attains top speed and there are no leaks.