finally secured my 16GB Nexus 4. Tips / steps I picked up from various posts in this story:

- If you see "Sold out", just refresh. I had to refresh many times at certain points just to see "Add to cart"

- Sometimes the "Add to cart" just fails, go back to the product page and try adding it again.

- Once you have the item in your cart, don't click on "cart" again, because your can (and probably will) lose it.

- Even though "Proceed" is grayed out, you can (and should!) click it again freely.

- Make sure to use firefox developer web console, firebug, or something similar for your browser, so you can hit "proceed" as soon as the series of 3 http requests has returned. they looked like this for me (I had to enable "Log" under "Network" for firefox web console):

  [16:03:00.085] POST https://play.google.com/xhr/ce [HTTP/1.1 200 OK 70ms]
  [16:03:00.085] POST https://play.google.com/store/checkoutcart [HTTP/1.1 200 OK 450ms]
  [16:03:00.538] POST https://play.google.com/xhr/ce [HTTP/1.1 200 OK 59ms]

- You will see "Due to high traffic.." and "Oops! .." errors constantly. Don't give up :)