I had had the same idea when I was pregnant but there were several key problems that I concluded were unsolvable. I am both curious and skeptical as to how/whether you address these. (I am basing my questions on an understanding that yours is a registry and not a simple a wish list, as per your own description).

1. How to mark items as purchased. Relying on gift givers to "reserve" and "unreserve" items may be a step up from restaurant reservations but that's not saying much. I wouldn't be surprised if your honor system led to close to half of every registry ending up in a black hole of forgotten reservations.

2. How to keep up with out-of-stock items and price changes. Baby registries usually span months, so how do you deal with outdated information or dead links? From my own Amazon baby registry experience, about 30% of my wish list went MIA by the time of the actual baby shower, when most of my friends and family made their purchases.

3. How do you deal with bad UIs. Drawing again from my Amazon experience, a lot of older relatives almost bought the wrong item because of all the "you might also like" items all over the screen. I wouldn't be surprised if a large chunk of your purchases got diverted by bad 3rd party UIs.

4. Varied URL standards. For example, some shopping sites get product options from user session so non-logged in URLs link to an item's default color and size. So once you are off to a 3rd party website, there is no guarantee the gift giver would buy the correct item.

As far as I can see you've built a wish list that you are marketing as a registry and I am not sure if that's a good thing. The key feature of a registry is that a retailer makes sure that the correct items get purchased (http://en.wikipedia.org/wiki/Gift_registry) so I think there is a broken promise there.