Service that allows you to search for the best offer of goods in existing online stores. Actually, first of all, for books - half of books are cheaper on Ozone, the other half - on Buks.ru, in the second - for components, electronics, home appliances, etc.
Technically difficult, the main difficulty is a robot to replenish the base. In principle, this is also beneficial for stores, you can try to negotiate with them in order to receive their offers in XML. The sources immediately get - Yandex Market, Buks, Ozone.
You can combine with customer reviews of companies.