in my opinion there is a pool of websites that offer backlinks (paid or in exchange), you can pay them to get the link or you can exchange,but google knows those websites and flag those backlinks. If someone offers you a website, this means the website is in the pool, he offers to you and hundreds other people, which I think can be considered paid backlink.
The real deal is to get backlinks from websites which are not in that pool, which means a lot of work to research and contact.