Acquiring links from reputed, authority sites using guest posts is a common practice in SEO. If you want to buy guest post, you need to know that not all services are same. When it comes to guest posts, you must focus on quality, and contents must be written for the audience and niche concerned, keeping traffic and social media influence in consideration.
Hire a company that has specific experience in your industry and is willing to share their work samples. Find how they generate content ideas and their expertise in understanding audience and generating targeted traffic by selecting right websites.