I took someone along with me (he knows a lot more about bikes than I do) to view a bike off ebay (local collect). Condition was perfect but size had been measured wrong so the sale was cancelled with no hard feelings, as no money had changed hands there was no hastle.
Next bike we looked at was a better bike, had had a bit more use but still under 1 year old. I bought it.
Both bikes were approx 50% of new price & under 1 year old.
Couple of months later a friend looked but everythink was selling at 75-80% (supply & demand), then he saw one & got it for 60-65%.
We had looked at bikes at large chains & LBS so had been looking for particular models. Going 2nd hand meant we could get a better spec. than our budget allowed us to going new.
Any bike is only worth what someone is prepared to pay. All of the 3 bikes we looked at turned out to be purchased under the 'ride to work' scheme so the seller wasn't losing out as much as it seemed based on how little the bike had been used. Then you have those who get a bike thinking it's a good idea at the time. Getting one of these & you may consider paying a tad more than 50% is worth it.