+1 Sgt Miller
The above is very correct.
I look at the condition of the weapon in question. I've brought some rough ones back from the dead and have a "Project Gun" on my workbench now...but usually what I find in pawn shops are dirty guns that have been uncleaned for years...sometimes decades. THOSE are the ones I tend to get great deals on!
Check the barrel first, inspect for pitting or other scars/tool marks.
Check the action, looking for broken/worn parts and any other oddities.
Then walk outside, sit in your car, grab your laptop/PSP, check prices & parts availabilty
Go back inside and commence to haggling!!
I never pay sticker. Not even on new guns. Never be afraid to haggle.
I also NEVER trade in old guns unless they are beyond all hope (like a Raven/Lorcin/Jennings/other junk guns), many people use a throwaway or two just for the purpose of being able to take a gun home RIGHT THEN instead of having to deal with the waiting periods. (YAY Florida! CCW, No Wait!! Alabama/Georgia - If you trade one in, No Wait!!)