I know for a fact Godaddy used to do this, I don't know if they still do (but would assume so). If you searched for domains and/or added them to the cart but did not purchase right away, they had their little known team of reviewers that constantly checked the list of all domains people were interested in purchasing. The ones that were deemed valuable were registered by an affiliated entity and offered in their auctions. My source for this information is a person who used to work there.