For one thing, describing an Automag accurately is tedious.
Saying that an Automag should go for $100 is like saying a football team should run a play.
Saying that a minty Classic 68 'Mag with Benchmark rail and Intelliframe, Level 10 bolt and CP on/off ASA should go for $200 is like saying that same football team should run "Flip right, double-X, Jet, 36 counter, naked waggle, X-7, X-quarter."
A lot more detail is needed to accurately describe what a marker has, and the composition combined with the demand for any certain part will dictate the market price. It's not something you can pick up in a few minutes. You need to browse some threads. The problem with thread browsing is that you also need to see some items being sold, because a lot of people take down their asking prices once something's sold and that makes it harder to gauge the market.