Depends on what you are going to sell.
Is it a real tangible product? (like selling computers, cars, etc)
Or will it be digital products? (like ebooks, software, etc)
For digital products, there are many great scripts out there.
Like my ebook store, that script is awesome and costs only $7.00.
Ebook Dawg for the best Ebooks | EbookDawg.com to see the script in action.
Already templated, hooked up with paypal, easy to add and delete products.