For an esoteric term that I tried, the results are about same for the top half of the page, but then were quite different for bottom half. Which leads me to believe they use Bing but do not completely rely on it (at least not to order the results).
The Bing API is actually open to anyone with a bank card but DDG has a specific deal to allow them to filter / sort results, to use ads not only from the Microsoft ads platform, and to skip the redirect link that records what the user click on.
Yes, other search engines can pay for access to Bing's results. DDG isn't just straight Bing results, though, it compiles results from a number of different sources (including their own), with Bing being the primary source.
"The results are aggregated from hundreds of sources, including DuckDuckGo's own web crawler, along with Bing, Yahoo, Yandex, specialty services like Wolfram Alpha, and others."
DDG's help website[2]:
"DuckDuckGo gets its results from over four hundred sources. These include hundreds of vertical sources delivering niche Instant Answers, DuckDuckBot (our crawler) and crowd-sourced sites (like Wikipedia, stored in our answer indexes). We also of course have more traditional links in the search results, which we also source from multiple partners, though most commonly from Bing (and none from Google)."
Clearly, they get their search results from a wide variety of sources including their own crawler. But it sounds like they do rely heavily on Bing.
That being said, the system is designed so as to not require trust in the front end. The JavaScript code is easy to read and ensures that queries are encrypted at the front end before passing thru a proxy to have its IP scrubbed. The engine then performs the query, encrypts the results and sends it back thru the proxy to the front end which decrypts and displays the results.
