I think all that you've said is true Eric. None the less, with enough
data it would be possible to see some trends - rather like the car
guides that you can get (Parkers in the UK for example).

I've heard several people comment that they've sold gliders for more
than they paid. I don't know whether that remains the case after you
allow for inflation (probably will in some cases - as you mention
prices go up and down).

I also observe that it's possible to buy a then state of the art
glider from the 1960s for £5K, from the 1980s for £20K and a new one
will cost you £100K. OK, not like for like, but my suspicion is that
if you buy a new glider and keep it for a long time, it will
depreciate. I'm a little curious as to by how much. Hence the
request for data!

Paul