Just in the comments section of my Bartlett piece alone, you can see how controversial the topic of a trade can be. I have been pondering the process of grading a trade and I have come to my own conclusions. I hope everyone gives their thoughts in the comments, as there is no definitive right answer.
In looking at a trade, it can only be graded at the actual time of the transaction. Often times people will look back at a trade and attempt to evaluate the trade based on the outcome. When evaluating a trade, you cannot look at a players performance after the fact to see who "won" in the trade, at least from a General Manager's standpoint. You cannot look at information that has happened after the trade to evaluate the GM's decision, since none of that information was available at the time of the trade.
For example, say the Rays were to trade OF Nathan Haynes to the Padres for SP Jake Peavy. This is assuming both teams Front Office has no inside information on future injuries and based the trade solely on their previous play. This would clearly be a completely lopsided trade, and would look like a total bargain for the Rays. Now, what if immediately after the trade Peavy lost all control of his pitches and his ERA inflated to 6+ while Haynes learns how to hit, gets his OBP to .370+ and steals 40 bases. Obviously, this is completely unrealistic, but try and picture this happening. Would the Padres become winners in the trade? Would we all of a sudden think that their GM was brilliant?
I don't think so! In general, this would still look like a great trade for the Rays, that just didn't work out the way they wanted it to. Regardless of how the players wind up doing after the trade, that is still a great trade for the Rays, because at the time of the trade we would have received more value. If you factor in information what happens after the fact, like Peavy becoming awful, you are grading a GM on information that is impossible to figure into his decision making process.