The best solution for creating an Ecommerce store in WordPress, in my opinion, is to use WP E-Commerce.

There is a good tutorial to get you up and running:

http://www.1stwebdesigner.com/tutorials/create-e-commerce-websites-wordpress/

*UPDATE* PHPurchase seems to be a serious contender to WP E-Commerce, with some users saying that it is already better than WP E-Commerce. So I suggest giving it some consideration as well before making your choice.

WordPress ecommerce shop Themes

Crafty Cart V2.0 (integrates with WP Ecommerce) – Free

Vanilla Cart

WPA Storefront

AppCloud

Shopperpress (theme + shopping cart)

Markettheme (theme + shoppping cart)

Artshop

eGoods

SimpleCartJS (v2 exists but not ported to WordPress)

WordPress ecommerce Plugins

PHPurchase

WP E-Commerce

Shopp

Further reading

http://mondaybynoon.com/2010/08/09/wordpress-custom-post-types-phpurchase-cart-part-1/

http://www.noupe.com/design/shopping-cart-templates.html

http://storecrowd.com/blog/wordpress-ecommerce/