Most major ecommerce sites actually pay for the PPC on sites like Overture, Google, DealTime, Bizrate, Looksmart, and even Findwhat. The rest are garbage. All the others are full of fraudulent CTR, and is not worth the time to deal with any of them.
We just started on bizrate.com about a month ago. Not bad results as far as ROI but we haven't seen all too many orders. More than second tier search engines but far less than adwords or OV for the same # of ads/products.
Not true, our PPC Search Engine is heavily based on repeat searchers. We have a few things that even Google and Overture can't offer due to their corporate greed which our users really appreciate. We do not have any affiliations with other search engines just a few good partnerships with productive sites.