What is the best way to set up a website where people can securely buy?

Best thing I can offer is a self-hosted WordPress site with the eStore shopping cart plugin which does the job well for selling digital products. Doesn’t cost the world and can be expanded to do affiliate tracking and members only sections of your website. Have a look at the plugins features here.

There are other platforms that do what you want: Salesforce.com springs to mind, but I’ve never used it myself. There’s also CRE Loaded B2B. I’ve tried it but find that good looking templates are hard to come by, even though the backend is solid.