multi-site cart

[eluser]summery[/eluser]
A user agent says which browser you're using, but it's not a unique string: people who use the same browser as you (admittedly, also the same exact build) will have the same user agent as you. Check out this site: http://whatsmyuseragent.com/ and this one: http://www.useragentstring.com/

You could examine the IP address; but there might be more than one user per IP address.

You can't use cookies to transfer data between sites, per my earlier post.

So I think the way to transfer information between multiple sites is using POST data like you initially suggested.

Good luck!