I'm not sure how Bioware plan to impliment this, but I came up with how I'd do it, after reading through this thread on the other boards.
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Reading through this thread, I've been wondering how I'd pull off an in-game DLC sales NPC. What I've come up with is to have an NPC merchant with it's own stall or shop, not someone that would be wandering around out in the world or following you and whatever else an NPC might do. Dialog would start out like any other merchant, "What are you selling?" NPC tells you they'll take a look to see what they have that you might be interested in, and walks over to something and does some sort of searching animation. While the NPC does it's thing, the game connects and checks for what's available, until it has the info for what's new, finds nothing, or times out. Once the check is done, the NPC comes back to you and says something, based on the result of the check.
"Can't find anything at the moment, maybe check back later." <= Can't connect.
"Didn't find anything new for you." <= You have everything that's available.
"Found something for you, would you like to take a look." <= There's something available that you don't have. At which point you can look if you want, or decide you don't actually care after all.
If you look at it, the camera moves to give you a view of the NPC on the left of your screen and a menu pops up on the right. The NPC will tell you a little about how they got the information or some random story about landing on cats and falling in wells and finding some interesting little item or something that may or may not directly relate but would be entertaining regardless. In the menu, you'd have a small image or logo or whatever related to a specific DLC and a brief description and price. If you decide to purchase, you get to fill in your info and the NPC gets to babble on about his love affair with turnips or if you cancel out, it just tells you that it hopes you'll check back later, the stock is always changing.
So basically, an NPC that you have to approach, that doesn't come after you, is found around other merchant NPCs so you have a purpose for checking that area through out the course of the game, never breaks character by actually talking about DLC and real currency, but still gives you the information that you need, is perhaps entertaining on its own with stories or jokes that perhaps get updated over time when it does it's checks, so even if you hate DLC, the NPC itself might still have value. I could see an implementation along these lines as working well within the game world.