Hmm. Sounds like you need two separate components, one of which is a spider and stores / indexes all the product information, and the other of which displays the info and allows people to buy items. I'd personally use Perl for the former, since it's easier to write heavy-duty code, especially page parsers, in, and PHP for the latter, since PHP is more efficient for high volumes of page views unless you have access to mod_perl.

Also remember that what you are designing now does not necessarily have to be able to stand up to billions of page views. Think of it as a rough draft. If you manage to get even thousands of people using your service per day, you'll easily have the funding you need to convert your code (which should be fairly rigid by then) into a more efficient language - or buy a new server with lots of memory that can brute force its way past almost anything.

Use whichever language is easiest to program in.