[elihu]

I think that eBay is already affecting price in a different way: buyers and sellers tend to use eBay price to figure out what a guitar is worth before buying and selling in person. Since used guitars are a risky purchase (there could be a lot of things wrong that don't show up in pictures), the eBay price is artificially depressed. However, since the eBay price is seen as the "fair market price", it deflates the price everywhere.

This could go the other way, as well: if sellers refuse to sell a certain kind of guitar on eBay, the price could become inflated.