People will always try to sell things for the best price that they think they might get for them and they don't always research prices before offering things for sale. (There's a charity shop near to me which often tries to sell secondhand clothes from, say, 'George' at prices much higher than they're still being sold new in Asda. It's simply a lack of research by the shop's manager).
eBay can still help out at times. I recently looked for a rare DVD on Amazon, to find that it was offered at £110. I bought it from a US eBay seller for a third of that price.