ask the sellers these things...
- what their sites' DR is (not DA, but DR, this is very important because DA can be easily manipulated while DR can not)
- how many OBLs (Outbound Links = External Links) their sites have (a site that has a lot of OBLs... or which links out to many other small / unpopular sites with each published article is considered a link farm - ie. site that's specifically created to sell links - therefore their links are usually of poor quality)
- how much organic traffic their sites get and how old their domains are (sites that are new and have no traffic might, or might not give you bad links, but sites that are aged and still have no traffic... well, it's a bad sign that you might get scammed, especially if the DR of those sites is low and / or the sites have very few pages indexed in google... or no indexed pages at all, that's even worse lol)
- where they place your link (usually, homepage links are the best type of link you could get, so you will want a link from the homepage of a good site, but if that's not possible at least get a link from an article / content page, it's still good if the site is legit. If the seller is only providing footer links, or other low value links you might want to pass.) .. Also at this point, no matter where your link is placed make sure that there are not 100s of other OBLs on that page because the link juice (site authority, domain strength, DR, or whatever you want to call it) will be split between all these 100s of OBLs so you'll get basically nothing. If the seller only sells 5-10 spots (ok, maybe up to 20, although 20 might be too much for a weak site) then that's good for all parties involved (his sites don't look like link farms, and his customers - ie. YOU - get value out of the purchased links)
- the sites in the seller's PBN are not copycats of each other. If all (or most of them) sites use the same theme, plugins, site design and structure, and even worse, the same hosting (well, IP anyway) and they're all interconnected to each other... these are signs of a bad PBN / scammer seller
- what kind of link (d0foll0w / nofollow) they give you
- how many RDs their sites have (although, sites with good DR usually have a decent number of RDs, as well)