This isn't a bad idea, and since it would be voluntary, then if its not your style, its not a problem. The fact is, you can't run entirely off trader ratings. If a person has never bought or sold before, it doesn't make them a scammer, although anything is possible. I wouldn't have been able to purchase my AEG, or sell the couple items that I have if people ignored those who aren't involved in large amounts of transactions.
Which isn't to say don't read trader ratings. If a person has negative or even neutral transactions, take that into consideration! It means they've had less than stellar dealings in the past, don't expect yours to be different.
Life is cheap, film is cheaper.