Each product you create in Seller Admin has its own, unique button code referencing that product's unique Item Number. Most of the product pages I tested on your site appear to have the same Add to Cart button code for just one product, so no matter which button is clicked, only that one item ever gets added to the cart. Make sure each product listing on your site is using the Add to Cart button code for that particular product:
http://www.e-junkie.com/ej/help.buttons.cart.htm
Your site appears to be built in WordPress, so you may be interested in this WP E-junkie plugin that makes adding cart buttons to your site even easier and avoids some common pitfalls pasting button codes into WP sites manually:
http://wpejunkie.com/
