I use PayPal for credit cards and other payments. I use mal's e
commerce and doodlecart (doodlebabies.com). I'd like to get PayPal to
be behind the scenes (seemlessly integrate into my site so that
visitors can pay directly on my site).

Requirements:
1) EASY front-end admin system where I can add/delete/edit pages and products.
2) Web-based storefront so I can work away from home.
3) Must integrate seamlessly with PayPal.
4) Must have extensive database capabilities.
5) Must have real-time inventory.
6) Must be able to offer coupons and gift certificates.
7) Must not change the look of my site! 
8) Must be linux compatible & php.
9) Must be able to suggest other products.
10) Must be able to have size/color/feature options for products.
11) Must have the ability to add-on a feature to the product for extra money.
12) Must have shipping integrated.
13) Must have html autoresponders available.
14) Must have an affiliate program.