There s definitely validity in trying to buy your own app, simply for the licensing service.
I tried to do the same thing, and received the same error on the server. My purpose for buying my own app is that even when I install the signed .apk file on my phone, the Licensing Verification Library that I use to check License validity does not cache licensing checks UNLESS it is purchased from the market. So there was absolutely no way that I could test the true Licensing experience that users would have on there phones before publishing!
In my case, my brother bought the app right away and tested it, and it worked great coming from the market, where it works a bit differently on my phone. Because of this, I have to wait for it to do the License check every time I run my own app. This wouldn t be bad, except for the fact that I can t run my app when I don t have a connection, such as airplane mode, because it doesn t cache the successful licensing responses!
I would really appreciate Google changing it so you just couldn t rate or comment on your own apps, but that you COULD purchase them. After all, you re actually losing money by purchasing it, since they take 30% for the carrier fees, etc.