Many domains that expire still have traffic one way or another, it might be from rankings in Google, social media sites, links from other sites, or many other various sources. So if you want to build up some cheap regular traffic, you can search for expired domains with the keywords most related to your site. You then purchase these domains, and simply redirect the traffic to your site.
Let’s say for example, you have a Las Vegas tourism web site. Perhaps you make money from selling Las Vegas shows, hotel bookings, and other various Las Vegas attractions. So you would do a search for expired domains using the keywords “LasVegas”, you could actually do the search “LasVegas las-vegas Vegas”, which would in turn yield a result for all the domains containing either lasvegas, las-vegas and vegas.
If it’s traffic you are seeking, then you could sort these results by Alexa ranking, order by the lowest first (for Alexa rankings, the higher the traffic, the lower the ranking, i.e. Alexa ranking of 1 gets the most traffic in the world).
So go through your list thoroughly, the top ones will have the most traffic, so check them carefully, to see how relevant the domain name might be. Once you see a domain that could be very relevant to you e.g. LasVegasTravelGuide.com, then you will want to know what this site used to be in the past. So go to Archive.org and see what the site used to be the past. If the site looks like something related, and relevant to what you are offering, then its likely the visitors from here will also like yours.
If you have alexa ranking between 1m-2m it could get nearly 100 visitors a day. An Alexa between 500k-1m might get 300 visitors a day. Alexa around 250k might get 1000 visitors a day. These numbers are very rough but gives you an idea.
In addition to the extra traffic, if you redirect the domain with a 301 redirect, the links might also pass on, adding further to your link count, and authority. It could also replace all the expired domain site’s rankings with the domain you are redirecting it to. In my experience this works only some of the time, no idea why not every time.
So what have we done here? There was a site people were visiting, only to be met with a 404 error, and essentially completely waster their time visiting attempting to visit this site. In addition to that, there were site’s that were linking to this non-existent domain, creating a bad visitor experience.
Now you have redirected them to your informative site, where they can get all the information they were seeking, and possibly even generate yourself a sale. A win/win solution all around.