Don't browse PBN/MCB/TECH PB for pump guns. You won't find them at good prices on there.
I find a lot of good deals on ebay and Customcockers. But more importantly you need to know what things are worth.
I would say start here: http://paintballgateway.com/wgp.html
but don't buy from there. The main difference between their guns are econo kit(1 guide rod) vs deluxe kit(2 guide rods), drilled vs undrilled, on/off asa vs non-on/off asa.
This at least sets the prices for you. For used guns a good price is 1/2 of the new price.
Just to give you an idea:
NOT MY LISTING but I saw this earlier today.
I think 160 is good price, even 180 is a good price since it is not drilled, has dual guide rods, has an on-off asa and has a clamping feedneck
I found some great deals on Craigslist. i.e. I found the following gun for $150. But that was because I knew what I was looking at. If you don't know what you are at, I would stay away from Craigslist.
*edit* 1 more thing the most common bodies that people want to pump are the 2000-2003 vertical feed and 2004 prostocks. If you can find one in decent condition for $60-$70 slap a CCM Deluxe kit you will be good to do for a long time.