35 on a trailer is a very fair price,
seller willing to allow test ride, good sign.
when you get there, before you start it, feel the engine to see if its cold, push/pull on the engine and see if it wiggles (bad mounts) look for rust on crankshaft or water or oil in the hull, signs of leaks.
Bring a good LED flashlight to look around the engine bay and the pump area, 90% of the people forget one.
you want it to start w/o hesitation, and rev up to mid throttle again with no hesitation (on trailer, and on water) Make sure gauges work and are readable (they can be pricey)
check bottom of hull, sides for signs of damage, look at pump area carefully for signs of corrosion, white (salt)
when on the ramp after test ride, keep it on the slope and pull the drain plugs, see if water comes out.
Ask seller for any/all receipts they may have, ask what oil they used if they know.
Expect to invest $5-800 into the ski after purchase to fix anything you don't catch on the inspection, if its less you'll be happy.
Doesn't hurt to ask about cover, life jackets, tow ropes, spare plugs, etc to sweeten the deal.... many sellers will hold some extra stuff back just in case your low-balling, < I know I do. (I removed my spare tire, jackets, covers, emptied the storage bucket on my last sale just in case the dude decided to get cute, he paid full price so after he paid me I said "hold on, here's some extra stuff that might come in handy"
Have him show you how to flush it, maybe because you need to know, but more importantly because you need to know if he knows. I had one about 6 months ago and it looked pretty good, I asked him if he flushed it after ever ride and he said "of course" so I said "well, i'm not sure how you do it, can you show me?" He froze, apparently he wasn't quite expecting that and also apparent that he had never flushed it, I walked away.
BRING CASH !!!!
you'd be surprised at how many people show up and don't have cash, its possible he's accepted a 3K offer and the buyer didn't actually have 3K, so rule #1 money talks, showing somebody a stack of 100's carries a lot of weight.
Make sure you have the right size ball on your hitch, if in doubt, bring both 1 7/8 an 2" or ask ahead of time, its surprising how often that is a problem. Check trailer lights, do they actually work, are tires worn, rust on trailer and bolts, saggy bunks?