I think you are much safer if you are buying a 4 stroke ski. Buying a 2 stroke would really be a crap shoot without a test ride, compression check, etc.. Getting a mechanic to check things out is a good idea if you don't know your way around the ski, but there won't be any way he can tell you it functions properly, only that everything looks OK.
Here is a potentially even bigger problem of not riding the ski. Without taking a test ride, you will have no idea how that model ski handles. Unless you are familiar with the model you are buying, and have ridden one of that type before, I wouldn't make a purchase. You may end up with a ski that is too unstable, bouncy, wet, etc.. Here is an example, before I bought my ski, my wife and I rented several skis. I learned pretty quick that I didn't like the Honda ride (one of my top picks before I rode it), and my wife let me know that she didn't like the Yamaha VX (too bouncy and wet for her).
If you have narrowed it down to one or two specific models that you know will work for you, then how about writing up a bill of sale that has a clause that says that you have not had a chance to water test the ski, and the seller warrants the ski to a specific date (something reasonable when the weather would permit a test ride).
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2008 Yamaha FX HO Cruiser
2010 Hurricane Sundeck 2100
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