You want to check if the sales copy claims match with the samples.
I sell PBN links.
If you want to buy PBN links, I suggest you put samples through similar process I went through when I was picking them for the network.
As a beginner though, look at the
major things:
content - is it well written & original?
referring domains number - (if there are supposed to be any) number and clean source is important
design - does it look like garbage or something an actual human made?
OBL - are there any other links already on the page, how many?
formatting - are posts just big, ugly piles of text with links or are there things like: sub-titles, internal links, images, bold / italic, quotes video embeds