About #1 ...
I'm not sure if you are talking about the owner of the ecommerce site?
The owner of the site can create a PHP script that will render an XML file
each time it is executed. Any user can activate the script when they wish
to read the XML file. An XML file is what an RSS feed is. This method ...
I suppose it could be considered an API (application programming interface).
But, a user just can't get a live XML feed from any site unless the site owner
has create a script that does that. Usually, an XML file (RSS feed) is generated
and cached (not in real time).
Alternatively, a site owner could generate an XML file each time an item is added
or removed. This would create a current XML file, but not necessarily "live".
So are you talking about this from the site owner's perspective, or a user perspective?