Hi Luba,
Not sure if this is an option, but here is an idea I just thought of.
Why dont you go ahead and get your site completed, since you have already paid the money, and use it as a standard site, and utilise the features it does have. Then, create a subdomain, such as
www.shop.mydomain.com or
www.feed.mydomain.com, and use the software that Linda suggested for datafeeds?
You can template this to look like your main website. This way you can use your main site to write articles, take site registrations, write a blog etc, and then use your subdomain to create a feed that you direct all your users to.
It is just an idea, but I think it might be a bit better than throwing your whole investment away!
Your other alternative ofcoarse is to find another system that does this all for you, which would require a whole new investment.
Hope that helps a little!
Cheers