Hello, welcome to JustAnswer.
A good running WC starts on the water without having to do anything special. A WC that has idling issues for what ever reason often has no problem when it is on the trailer.
Boats run differently when they are not on the water. Usually they run much better. This is because when a boat is in the water there is a load on the motor just pushing the exhaust thru the water.
A jet boat (which is what a jet ski is, has a whole lot more load on it because they are always in gear, even in "neutral" that jet pump is pushing a lot of water.
So, a motor that is wore out, may sound and run great on the trailer, and not even start on the water. This is the reason that you should NEVER purchase a used boat that you can not test drive.
Jet ski's that are over 10 years old are very old as compared to a regular boat. If they are 15 years old I wouldn't look at them no matter how good the price is, and $1000 is not a good price. $500-$700 is all they are worth. for the 750.
There are a number of issues that could be the cause of the no start problem. Most likely is low compression.
I would take them to a shop to have them looked over and get an estimated of their condition. Honestly, they may not be worth the repair cost.
Had you asked before you purchased them I'd have advised you to walk away. For your first time you need something that is going to be more reliable than these.