Aidan, I agree 100%, and its not just keyboards. Five years ago, you could place an ad with stuff for sale, set a reasonable price, and people would buy. These days, you get want ads telling you how much people are going to pay, and the prices are pitiful, 25% of list price in some cases.
I saw an ad yesterday in my local press, wanted, good amplifier for £50. These people are living in a dream world, they want stuff for nothing.
I have an absolutely mint Les Paul standard, worth £1300 of anyones money, and I got some troll slagging me off because I wouldnt let him have it for £800! I am putting my SK1 on eBay tomorrow, had it on Reverb for three months, not even a smidgen of interest. I have at least a dozen guitar pedals, a banjo, a 12 string, a two manual original Crumar Mojo and a HK Lucas Nano 300 all in mint condition, sitting in my spare room, and they will stay there until someone offers me a reasonable price. I dont want a fortune, just whats fair.
The bottom has dropped right out of the market for used music gear, at least in the UK. (Rant over)