pretty much what you said look for obvoius signs of discoloration scratch marks on the leeding edge. and your best bet is to fly it if possible. this will give you an idea if the lines need tuning.

but i would definatly check out another couple of kites and demo a few before you buy man, you might find something you like alot more than the convert and your 9 can become your favourite toy :) i own 4 different kinds of kites lol it takes about 15 min of using one if say i hadnt been using it in a couple of weeks to get used to it agian then its fine.

you could always buy my 9metre 08 best nemisis which is pretty much new for 1100 or my 08/09 hadlow pro 9 metre also 1100.. but that ones a c kite so not really reccomended unless you want that kind of kite :)

If you like the 12m of the brand you are using, you will likely to love the 9m. The problem starts the other way around: you love your 9 and the 12 may prove to be a pig.
Solution: fly it before you buy it.

Check where the pullies run also, another indicator of the amount of use, but if it has been used for lessons, you know it has constanly been crashed, day after day after day, get one from a normal kiter, that has had a normal amount of use, probabily still day after day, but by an experienced kiter (or at least a familar and improving kiter). Not a new newbie each day.. 2 cents

i would see if i could pump it up and leave it for a few hours and see if it goes down. nothing worse than a slow leak