The long standing rule of thumb going back decades is that you don't trade within your division. You don't want to help your competitors get better and don't want to have to face the player you traded away over and over every season.

I think that is extremely outdated and faulty thinking. Since FA arrived, teams have no choice if a player ends up with a rival. Take the AL east for example. Baltimore had Mussina who finished his career with the Yanks. Boston has Ellsbury who's now with the Yanks. In fact, I think some of the prevailing correctness likely dates back to the Red Sox selling Ruth to the Yanks.

But, now, things have changed. You have FA, interleague play, a more limited number of games against teams in your division, the Wild Card, etc.

Now, I think the smart play (assuming needs align) is to take away your competitions future with your present if you are a rebuilding team.

Take the Braves and gNats for example. The gNats just shipped out 3 relatively high end pitchers for Adam Eaton. They hurt their future by trading for their present. But, let's say that the Braves traded them Inciarte instead for some portion of what they traded away for Eaton, say Giolito, Fedde and Dunning.

The gNats got to keep Lopez in that scenario but got an arguably lesser talent in Inciarte instead of Eaton. But, they got their CF in either case. The difference in the second case is that a direct competitor (the Braves) took part of their future in exchange for the Braves today. Now, obviously, you don't do that if you are the Braves IF you intend to really compete with the gNats on a short term basis. But, who really believes that the 2016 Braves will challenge the 2016 gNats for the NL East? This is the gNats window. It is closing and will likely be closed when Harper goes elsewhere. They might be able to overcome that IF they have a minor league system full of talent to replace what is lost. But if the Chisox have that talent, it will make it tough. If the Braves have that talent though, it would work two ways, make the Braves better then and make the gNats worse.