The best thing you could do, if you actually care about privacy and not just $$$, is to open-source the entire search index db and accompanying webserver software, making it easy for users to setup their own local instance of DDG which is actually auditable. Additionally, posting a notice on-site which notifies your users that their searches may be recorded and tracked in spite of what the privacy policy says(due to the USA jurisdiction of the company making it susceptible to National Security Letters and secret gag orders) would be the right thing to do.