You can safely buy backlink building services from trustworthy SEO agencies, however, you can't just buy backlinks outright. You pay reputable agencies for outreach on your behalf, while other disreputable agencies use private blog networks and other black hat techniques. Good backlinks are on high quality websites relevant to your niche. The link is in the content, not on a sidebar or in the footer. Poor quality backlinks are on low quality sites, on directories or spam comments in forums and blog comments.
It's almost impossible to buy links from government or education websites or high authority news sites, but an agency can help you get one. They create stellar content and market it to these sites, which is something many webmasters can't do or don't have time to do. For example, an agency could create a well-researched list of the highest paying jobs in a relevant industry to your niche. Next, they can market the content to college professors who teach the subject or others who write about industry. Their content creation and outreach is what you're paying for, not the link.
When buying backlinks, you get what you pay for. If you buy 100 links for $5, you'll get crappy links which will hurt your site. Paying an agency for $99 to build links for you should get you the highest quality backlinks if they are a trustworthy agency.
You can pay someone to earn links for you, but you can't pay outright for the link. Paying outright for links is dangerous. You may incur a Google penalty or the site you purchased a link on may get one and then remove your link. Additionally, backlinks should bring you traffic. If your link is on a spammy website, no one will follow the link. Earned links on quality sites in your niche can bring you qualified traffic as well a better ranking in the SERPs. For more information click here https://www.reddit.com/r/SEO/comments/ea2rzd/buyingbacklinksfrom_agencies/.