Most of the guns on all three of those sites seem way high, to me anyway. As an example, look on Gunbroker, go to, say the S&W revolvers, and see how many of the guns there have Zero bids. A lot of those guns have been listed there for a long, long, time.
Then again, if you go by the Gun Digest Standard Catalog of gun prices they seem to be more in line, conservative, if you will. which is good, unless you are selling!