As Tony said as an immediate response to this post, its a simple matter of economics. The second hand gat market is a freemarket economy, not a planned economy. This basically means, the buyer and sellers meet and establish a price which suits gives the maximum welfare to both parties. This doesnt change if one person offers say £50 for a Karnivore, because the seller will simply reject any offer that is too low. Eventually, after a series of offers on a product, a price will be agreed that is agreeable to both the seller and buyer. If this point (known as the free market equilibrium) is not reached, no sale will take place. It is as a result of many hundreds of transactions that people come to accept the rough prices that markers will sell for second hand. For example (please forgive me if Im wrong here, im merely trying to demonstrate) Id say that one could expect to pick up a second hand E1 Upped Autococker for between £150 and £200 depending on upgrades, condition, location etc. If somebody tried to offer £100 for it, there would almost certainly be somebody who offered £150 for it as there is demand for the product at that price. If the seller tried to sell it for £300 it is unlikely there would be any demand for it at that price...
The only way this will change is if ALL the sellers in the market place decided to raise their prices in line with one another. However, I am quite convinced that there would always be somebody willing to undercut the new market price. Also it would take a long time for the buyers to react to the change, and would also affect the demand for the product, if the price is getting near that of a new gat the buyer would be more inclined to buy that.
I hope that made some sense, and I also hope I got the average prices right too.