Insert it in the existing content of your page and add some relevant text before and after the link to make the whole article sound naturally.
If you don’t want to invent any text, just go to our client’s website, copy some text right from it and use it on your own webpage (feel free to rewrite it a little bit with your own words if necessary). In this case you can leave your existing article on your webpage as is, without any changes, and just add a new paragraph below or within the article with client’s link inserted in that paragraph.
Our client will LOVE such link because it will look natural and because it will improve his site rankings on Google much better. So such client will NOT remove it and will buy MORE links from you. Moreover, as a satisfied customer, he’ll recommend our system to his friends and colleagues so they’ll also come to us and buy links from you.
If you don’t want to do any of the above options, then at least add a few words before the backlink and after it. Then such link may look like this, for example: ‘If you want to monetize your website, we recommend you to sell links through MageNet.’ The backlink itself is ‘sell links’, but it is surrounded with a few related words, so such link looks more natural for everyone: our client, Google and your site visitors.