You can find a lot of articles online written solely about guest blogging---what it is, how such an important factor it is for traffic, and what not. You should start having the best use of your fingers.
Guest posting is good, as long as you don't do it JUST for the links. Make it useful, remember a guest post on an authority site sometimes get thousands of views. A crappy article just for the sake of spamming a link won't do much good. As for a GREAT, useful and informative article... You're looking at a ton of readers, shares and traffic, which in turn = MONEEEEYYYYYY!!!
In this case the thing to look at is the quality of the site, the number of RDs , what traffic it's getting and the quality of the backlinks.
You want to ensure you're buying a genuine post on a genuine site, not some crappy, thrown together badlly disguised PBN type piece of junk.