I think now there is a lot more volatility in the prices as stuff goes in and out of fashion. I think before the price guides were out, ebay was similar, an item could go for $500 one day and $12 the same day. It puts experienced collectors who can pick out good records for cheap in an advantageous position. The price guides had the function of fixing the price, to some extent. Now that there isn't a very current one (and that there are so many different scenes to document) it's like there's not a guide.
The funniest is Osborne's guide, which was written at the absolute peak of prices, the worst UK-US exchange rate, and even taking that into account, has higher prices than any UK guides.