I know that link selling was blamed by Google in the past. I also consider it bad to buy links to manipulate search engine ranking positions. The main goal of a link placed on one website that leads to another is a classic form of advertising. Is advertising bad ? Not at all, even google most profitable business is around advertising ( google adwords ). During the search engine optimization gold days, the myth that a strong sidebar link will greatly influence the position of a website in google. I always tended to believe that relevant quotations are the most important factor in google algorithm. The result of this popular belief was a rapid growth of link sales. Most black-hat SEOs didn’t wanted to diaplay the link to normal users, only to search engines and this is why Google started to take action against paid links.
In the recent Google algorithm changes, less and less value is given to out of content links. But again, links are intended to let the visitors of a website to find out about other good related websites, and their main goal is to advertise a website. Wembasters are still buying websites, but if you only have a small niche website it will be hard for you to manually attract advertisers. A link is usually sold for anywhere between 10$ to 100$ a month ( for small websites ), and a product review ( also known as paid blog post ), can be sold from $20 to $200 ( websites build for selling paid blog posts does not count ).
As I said, for a small website, is is practically impossible to sell the links. You can let other webmasters know that you are selling text link ads on related forums, webmaster forums, or you can join advertising brokerage websites such as LinkVehicle. They offer you good revenue for you sold text links.
Disclosure: Some of the links in this post are "affiliate links." This means if you click on the link and purchase the item, I will receive an affiliate commission