That depends on what S/W you want to run.

A typical open source solution such as ZenCart or osCommerce requires PHP, MySQL and SSL. Choose a host with servers in the country you want to sell to, especially if you have a non country specific domain such as .com, .biz, .org etc. Bandwidth obviously depends on how busy you expect the site to be but typically about 3GB/m should suffice for most fledgling stores. Don't be tempted by the "deals" you see offering "unlimited" bandwidth. If you have "unlimited" bandwidth then so will everyone else they have crammed on the same server as you.

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Lawrence