Selling links isn't exactly ideal (White hat SEO). But if you think of it as guest posting, then it can begin to make sense.
Think of it this way, if you acquire relevant links to your website, does it really matter how you got it? They are relevant simple.If they weren't relevant, the sellers wouldn't have sold it to you because linking out to you could potentially hurt them too. But then again, if the seller is a novice, this might be a challenge.
So to answer the question: Should I sell links, or even should I buy links? From where I stand, the answer is yes, if you know exactly what you are doing. And by this, I mean if you have a basic understanding of SEO. If you know the kind of links you need, buy them, and if someone approaches you to sell, sell. After all, you are already linking out to people for free on ONPAGE SEO
So where do you buy links? Head over to DripMotion.com, where blog owners come to buy and sell links to grow their blogs. They can also buy other stuff like graphics from the market place. If you haven't registered already, what are you waiting for?

So you wanna start selling links

Of course yes. If you will buy high quality backlinks to your site and make right SEO works with your site, Google will improve your position. Alternatively, you can start selling links if you have popular website with good content. I recently started working with https://linkdeploy.com/ they provide very cool possibilities to earn on this.