I have one or two competitors (in the UK) in my field who buy expired 1 - 8 year old domains on random subjects (SEO, travel, health you name it) and they are in the printing business and they stick 1 - 2 articles (unrelated to what was on there before) on these and that's it.
I think they stick with PA and DA above 30 and most have 10 – 100 links so well used expired domains, hosted in the USA and most have different Ip’s although they now have that many (over 70% of their backlink profile) that some have the same ip.
On further investigation none of the blogs have any contact details but it does look like they have been a little smart here and added content to the about us (similar to I use to run xxx but now do xxx) also they have one or two tabs with content on (article length) that is on the same subject they use to do and the titles are all the same content.
So basically they are finding expired 1 – 10 year old domains that have only been expired (from what I can see) 6 months max and putting 1 – 2 articles on the home page in relation with print (maybe adding a third on the subject the blog use to cover), add 1 – 3 articles via tabs at the top on subjects the sites use to cover, registering the details via xbybssgcf@whoisprivacyprotect.com and that’s it.
They have been ranking via this method for the last couple of years (through all the Google updates) and still do extremely well.
Does Google not have any way to combat link networks other than the stupid stuff such as public link networks, it just seems that if you know what you are doing you get away, if your big enough you get away with it but the middle of the ground (mum and pop sites) get F*** over with spam pointing to there site that no spammer would dream of doing anyway?