Domains with traffic always have keywords list, by this keywords usually comming organic traffic from search engines. As minimum must be verified alexa ranking included, and also listed if there fake alexa rank.Sometimes it can be really expensive keywords which we can use in google adwords and grow at it. Good algorythm can give answer of minimum visitors number per month, based on alexa data or google data.
Who thinking seriusly about make a SEO, buying expired domain name with traffic and inbound links.
Most popular auctions with expiring domains are at godaddy, namejet, snapnames.
Search for expired domains name its very easy if you have domains list.
With a bit luck, you can drop good one with PR5 cheaper like $200, just need to know which domain have not fake pr.
If expiring domain are listed in major directory, then easy have traffic from them, also still received a bit page rank, especialy from DMOZ, where in some categorys high pr.