When seeking for blogs as a part of guest posting services, you will encounter many blogs which are built on dropped domains and Private Blog Network or PBNs. The former is the expired domains bearing a link profile which the SEOs buy, upload low quality content and then sell away the links. This is a money grab opportunity for an SEO expert who aims to selling more links.
There is nothing wrong with purchasing an old and expired domains for your requirements, but it takes a lot of hard work to make it a legitimate website. Additionally, you will also need to ensure that the old links are immensely affecting the site rankings.
PBNs are websites that were built privately, it means that Google shouldn’t be aware that there is a single owner. The gist of PBS is to act as a source of backlinks for the main website.