I browse used bike listings a LOT...several times a day. Kind of a habit from the months it took me to collect parts for my bike. I've bought and sold 6 bikes in the past 3 years, so I've dealt with both ends.

How good a deal is depends on how patient you are. I have all the time in the world, and I've gotten top tier bikes <30% of their original retail price. It seems that the more expensive the bike, the better the deal because the market for high-end bikes isn't really that big. Once every few weeks I'll see a XTR XC bike for <$800 USD, or a DuraAce road bike for $1000. These bikes are <5 years old. If you are an average sized male, then you have a lot of choices.

To me, 50% is a bad price. Ebay is a TERRIBLE place to buy bikes, buyers overbid frequently and buying a bike unseen is risky. You're much better off with local buy/sell sites, if there are any for your area. The more people that an ad reaches, the higher that bike will sell for.

Bottom line: buy locally, sell online.