We started off adding most of the major e-commerce stores. As to how we decide which sites to add, well that depends. We usually add sites based on what our users seem to be asking for. More sites means more ability to do interesting queries like comparing the prices between store A and B. Since we have the products disambiguated, it is much easier.

Or if we find that the existing data that we gathered from our existing sources for a certain vertical could be improved, we add new niche sites to augment the data that we already have.