While the law says no one is allowed to sell marijuana, Target appears to be getting around that by selling the bag that the marijuana seeds come in, rather than the marijuana itself. This is actually how the Jack Daniel's Distillery in Tennessee is able to sell whiskey, as it is located in a "dry" county where alcohol is illegal -- they say you are buying a commemorative bottle, rather than the whiskey that is inside. I have been there, it is pretty cool.
I guess we'll see if this interpretation holds up, but the company is big enough that presumably they have lots of lawyers who have analyzed the issue.
In addition, the DC law states that no one under 21 is allowed to have marijuana, and no one is allowed to carry more than 2 ounces, so they are making sure not to sell more than that to any one person. However, with each friend you bring, you can buy an additional two ounces -- so if you are shopping with one person, you could buy 4 ounces, and so on. I hope this doesn't result in huge lines as people bring tons of friends shopping with them, the lines there are bad enough already.
The 2 ounces also only count as one item, so you could still use the express lanes.
They will also be selling marijuana paraphernalia in a special seasonal section, which is going to replace the Easter aisle. This will include paraphernalia like pipes, bongs, one-hitters, rolling papers, screens, black lights, Girl Scout cookies, Planet Earth DVDs, and those lights that change colors or whatever.