I love buying things from ebay because it's a highly competitive marketplace. You can find things you can't find anywhere else, and you're almost guaranteed to get the lowest possible price. It also gives you direct access to foreign vendors who can give you better prices than buying retail.
Of course, like anything else, there's due diligence in researching the sellers. Even buying from a reputable source can sometimes lead to an unsatisfactory transaction, though that can happen even when you buy locally. The only advantage to buying locally is that you "know where they live", so to speak.