This is something I was busy with the last couple of months: ScopeHunter.
It’s a internet search engine for optical instruments. It checks quite an amount of auction- and biddingsites and internet forums related to astronomy and photography with a buy/sell section.
If you’re looking for a telescope, microscope, webcam or a DSLR camera, you don’t have to search the web yourself.
ScopeHunter does all the work for you. You can even subscribe to a RSS feed to get regular search results.
Happy hunting! 🙂