There are open source E-commerce packages that can be customized and could be made to offer the features you mention. But creating such customizations is well beyond your current experience. You would need significant skills and experience in both PHP and JavaScript, and at least a passing knowledge of Internet security.
You could certainly start with a package like Zen Cart or osCommerce and rely on their basic configurations at the start. Both packages have a large support community that offer extensions which provide special features, and the dynamic pricing feature might be available for one or both.
What you don't want to do is invest a lot of time and money in creating a site without first developing a solid plan for setting up your business legally and financially, and for attracting traffic to your site. Find out the costs of things like hosting services, SSL certificates, business and sales tax licenses, and merchant credit card accounts. Learn how to build traffic to a website. It's dirt simple to build a simple website these days, but these are things you need to know beforehand in order to really have a chance to succeed.
