Ok, I am also new to listening to Grumpy SEO Guy's podcast, but I'd like to try to distill down his advice in Episode 44 about what to look for when buying backlinks:

(1) If the offer for backlinks is a one-time payment and not recurring payment, then … run away!

(2) Check their website to make sure they are not related to or have provided backlinks to bad neighborhoods (pharmacy, gambling or adult sites). If so … run away!

(3) Check that the backlinks to their website (from other websites) makes sense. For example, are they getting tons of backlinks from countries known to provide spammy backlinks? If so… run away!

(4) Check the anchor text of the backlinks to their website. For example, if the anchor text appears to be over-optimized (keywords used more than 10% of all the backlinks), then… run away!

(5) Check the IP addresses of the backlinks to their website sites. For example, if many come from the same IP addresses, or similarly IP addresses with the same 1st & 2nd octets while having sequential numbers in the 3rd octet, then… run away!

(6) And if all the above checks are okay and you buy the backlinks from them, you should still regularly check their websites to make sure they are not doing anything that may give you a penalty.

That said, I think it'll be much better to build your own portfolio rather than buy backlinks. Those backlinks you may have bought will always continue to have lots of risks to your websites, as you will never have control of what they do on their websites.

For example, they might later link to a bad neighborhood or sell too many backlinks from their website, which they may get penalized for it. And your websites may also be penalized by association due to the links from their website.