About mutli sites, it can work but I don't recommend it because there are issues. You could maybe do it if you are only using PayPal, but to use Google Checkout or Authorize.NET you need an SSL certificate. Only 1 SSL certificate can be bound to port 443 for a given ip address so it poses a problem for sites all using the same ip address as the SSL cert also has to match the domain. It can be worked around if you own the server because you can point more than 1 web site to the same physical folder and share a single installation that way but with different ip addresses for each site therefore allowing each to have an SSL cert.
For serious ecommerce I recommend a dedicated server I would not feel comfortable myself doing ecommerce on shared hosting. Some people may do that but its not what I would do or recommend. Of course one could start out on shared hosting and if you start doing a good business then you can afford to move to a dedicated server and should in my opinion.