The main reason third generation processors are still quite expensive is because they are still relevant. They are still widely used in businesses across the world, due to their affordability and performance. Picture yourself the owner of a business that needs computers to browse the internet and make excel spreadsheets. Third generation processors have more than enough power to accommodate for the average person, for the simple and medial tasks.
Another good reason for the pricing of the processors, especially on a site such as eBay, is due to the original price a person paid for the processor, and the generalized pricing across the web. If you were to sell an i7-3770 on eBay, you bought it for $300 and you see other people selling it for $180, are you gonna sell it for $100 or try to match the price most people are selling for?
Also, the i7-3770K processor is an extremely powerful chip for that chipset. It is actually more powerful than some processors from newer generations, and can be overclocked quite easily, allowing it to be even closer to the top range of processors in raw performance. I wouldn't buy this processor unless you have decent cooling + a motherboard that can overclock.
As others have stated, the i7-3770K apart, most of the third generation is falling strictly into obsolete territory for enthusiasts and gamers alike, and you would be better off getting a more modern system, as newer instructions are created for applications and games that your processor will no longer support. Additionally, games are being optimized for 6 cores currently, whereas the 3770K only has 4.