Amazon's Registry system works for non-Amazon items:
Below or next to registry items,
you may see a link which reads, "Buying this item elsewhere?"
Clicking on it will take you to a page which states:
"If you buy this item from another website, we cannot mark it as
purchased unless you tell us. Doing this will prevent the chance of
the recipient getting a duplicate gift. We will send you an e-mail
from which you will need to confirm that you will be buying this
It will ask you for
your name (optional, although if you leave it out, we may not know
who the gift is from), your e-mail address (required), and the
quantity to reserve (defaults to 1, but you can change this field).
This is helpful in case you find an item on another site or in a
physical store, or if you do not have an account with
Amazon or the site(s) where the items are actually for sale. If you
do not fill out the form, the item will continue to show as
"unpurchased" to other guests.