Just catching up with the thread, had some hectic couple of days
1. The OP has kindly agreed to cover half of the cost of repairing the coilovers, which is fair enough in my view. KW UK charges £120 + VAT (20%) per corner, which is £144 = $232 per corner. Shipping and return shipping fees will also apply, therefore the final bill it's going to be nasty.
2. In the telephone conversation I had with a senior KW UK technician, I've been asked to test the following:
a) set stiffness to hard, push the piston down - it went down very easily on rear shocks, one sign that tells it is failed; front shocks seemed OK (with the spring and OEM top mount removed)
b) set stiffness to soft, push the piston down - it went exactly the same way, very easily, so point
a) was re-confirmed
c) with stiffness to hard/soft, pull the piston up - it went up easily on rear ones, and the other way round, same rate, like I did not change the stiffness at all; front shocks, seemed ok
So after talking to the technician we concluded to send them in for rebuild (they will then be forward to KW factory in Germany), and whilst the front shocks seemed fine, I wanted to do all of them to avoid failing at a later stage.
2. The OP was helpful and has responded to my PMs, so please don't let this awkward situation and the bad luck to affect its seller reputation!
As a buyer, I consider this matter now settled, MODs please feel free to close the thread should you wish so.
Thanks everyone for PMs, advice and information, love this forum!