For carts....you can look at free installs of Virtuemart, osCommerc, CRELoaded, Zen Cart and others....CRE or Zen are good places to start....of course there is also eBay Prostores if you like that platform.

"but if I pay the $299 'lifetime' fee, where do I go from there? "
You would need to contact the suppliers directly to setup accounts so you can order products. Next you would need to manually upload the data or find a source for linking your site directly to the supplier product feeds to upload the data for you.

Also...here are a few supplier sources to help you get started with products if you are looking for some products:
http://moteng.com/
http://2buypcs.com/newpcs/index.aspx
http://www.dbldistributing.com/
http://xsdepot.com/