It's all relative. You might buy for £2800 and sell for £2200 (by which time a brand new one may cost about £3500*) in which case everyone is happy so long as the old one is still in tip-top condition and comes with all original packing (also in good nick) and paperwork and cd transit screw etc.
I bought a Nait 5i and CD5i ex-demo for £1440 together (10 percent off full price at the time). Two years later they went for a total of £1130.
A NAT05 bought used for £450 sold for £450 and a set of n-Sats bought for £450 (ex-dealer) sold for £420.
None of that included postage which was all charged on top.
*Extrapolating from how the price of the original Naim Uniti went up during it's product life from £1995 to £2500)