I would be very surprised if search engines used straight traffic as a ranking signal. Most websites get most of their traffic from search engines. If search engines used traffic as a signal, popular sites would only get better rankings and more traffic in an inappropriate feedback loop.
There are other sources of traffic such as links. Search engine bots crawl for links and use the weight of those links as a ranking signal. In the case of links, using traffic as a signal as well would be double counting.
Social media can be another big source of traffic these days. Google is trying hard to get Google+ to be the number one social network in part so that they can see this data. Search engines might try to use social media traffic as a signal if they could get the data. It seems feasible that this data could be bought from social networks.
Advertising traffic is another big source of traffic. Google has specifically said that spending more on their advertising network will in no way influence rankings. It wouldn't make much sense for search engines to rely on bought traffic to rank sites higher anyway. I can only imagine that the search results would be filled with Viagra sites if they did that.
The last major source of traffic would be type in traffic and bookmarks. Again, this would be a reasonable signal for search engines to use if they could get the data. In this case, the data would be much harder get than from social media. They would have to rely on browser plugins that spy on users or ISPs that spy on users.