the landing-page would need to include a secure e-commerce method to purchase the item.
google has rather strict rules for submitting marketplace items -- such as a shared common site.
for example, the marketplace is responsible for all feeds, each merchant (marketplace-seller) must be given a sub-account under the marketplace-account, prices must be fixed, duplicate listings from multiple marketplaces are generally not allowed, items may not be submitted on behalf of any merchant with existing active listings in product-search, a proper image must be submitted, proper global-trade data or an exemption is required, etc., -- in addition to google's general rules and policies for feeds, items, and websites.
if only one submitted item violates google policies the sub-account and marketplace can be disapproved.
generally, you submit the marketplace items on behalf of sellers and use the same upload techniques available to individuals -- feed file, manual-upload, scheduled-fetch, ftp, or api -- within the context of your multi-client account.