[Tcoat]

Well let's see here...
You would need a fairly large number of the item to be able to get any sort of reasonable "average" price. A sample size of at least 6 and the more the better.
Items would have to be exactly the same condition or else any pricing set would be meaningless.
Local supply and demand is still going to set value. What a part costs in NorCal could be radically different than Utah.
The age of the ad could skew prices way up or down which would again be reflected in the average. What may have sold for dirt cheap in 2014 my now cost more or vice versa.
There really is no way to determine a "market price" from listings on here. If there is something you want and the price looks good to you then buy it. If not check out Ebay and see what they are selling it for.