[Demisis_P2P]

It's an okay idea in theory but it wouldn't actually do anything.
Players rarely sell anything of value for straight cash.
Would be way too easy to game the system by getting a friend to keep trading you their M4 for $1 so that the price looks lower so you can buy one.
Even if you only limited it to only recording trades to once per day per item per account and only if the sale price is within a threshold of an existing average sale price, groups of people (parties, gangs) would still be able to influence the prices of items fairly easily if they wanted to.