I quite like the idea of that (or something similar - I've heard of Magic, but don't know anything about it). As an alternative, I've seen there are often questions about shopping carts - I'd be quite interested in this, in fact I've done a simple version for some stuff I'm involved in - but I think it would offer some useful stuff:

• CGI
• cookies - good idea or not, alternatives
• db back system - i.e. mysql or something similar
• user interface issues

Of course, again this is only a suggestion - and I'd be interested in some alternatives.