The problem with that is the market is always fluctuating.
for example prices on used weapons will be a bit lower before Christmas, People need shopping money.
Prices jump every time the BATFE or congress makes a move on the Guns.
So your Glock that might have sold for 700 during a panic might only be worth 400 during an economic crunch.
Honestly I Find people are scaling more with Gunbroker than anything else.