SaltySinus said...
Check the pig tails for wear as these give a good indication of condition and time spent in sun/salt water.
True but pig tails can be replaced. One of the cheapest things you can do to make an old kite look better for sale!
Scratches or fraying of stitching on the LE is a tell tale sign. When kites have been dragged or crashed on the beach it's the LE which takes the brunt.
Probably the best thing you can do to stack the odds in your favour is to pick your seller. Ask the seller what size kites they have in their quiver. If they say "just a 9m" then you know that 9m has been used in all conditions each day of the week. If they have a 7/9/12 combo then the wear will have been spread relatively evenly over all kites.
There are plenty of kiters (like me) who run 3 kites and upgrade every season or two. These are the people you'll be wanting to buy gear from. You can stack the odds further in your favour if the seller has a family and a demanding job, ie. not a lot of kite time!