Hifi shark is a good choice for a reasonably clear picture of what items are listed for.
What I found with the Audiogon bluebook when I subscribed several years ago is that average price is not weighted based on age. IOW, if an item were 2 generations old, the sales from when it was current production was considered the same as current pricing. Agon bluebook average tends to be over inflated. I am not sure if the average pricing method has changed or not for the bluebook.