I worked with a guy who was on Ebay hours a day. at any given time he could tell you the going rate on most car audio stuff on there and be within a couple bucks. he ended up just buying and resellling a lot of CA stuff just from constantly knowing the prices of stuff. he never got burned on anything because he only bought stuff from people with good feedback. i remember for a while last summer he literally had shit comming to his house once a day that he was buying and turning around.
i guess the moral is, buy shit from people with decent feedback and you will be straight.
I buy on ebay and resell at the local flea market. Best sellers I have are Powerbass 10's and 12's. I sell them for $50 - $75 each and my cost is $23 to $33 per pair. I sell Hifonics 12's as well For $75 each with my cost being $24 per sub. Nice way to make some fast cash.