You asked for the question that separates them. I don't think there is one, but there is a sequence, and it's cheap to run. This is what I do before anything above about $200.
1. Establish what you are buying
There are four products sold under the word "link" and the price ranges overlap, which is how people get confused. A guest post on a site the seller owns. A guest post on a site the seller has a relationship with. An insertion into an existing page. A rental. The failure modes are completely different — rentals disappear when you stop paying, which is honest; insertions disappear when the site owner does a content audit, which isn't anyone's fault; owned-site guest posts disappear when the seller sells the site.
Make him name which one it is in writing. Half the disputes I've watched came from the buyer thinking he'd bought permanence and the seller thinking he'd sold twelve months.
2. Check the page, not the domain
Domain metrics tell you what the seller has invested in his own inventory. The page your link goes on is the thing that has to be crawled, indexed, and read by something. Two checks: is the target page in the index today, and does the page have any inbound links of its own. A brand new post on an aged domain, with nothing pointing at it, sitting five clicks from the homepage, is a link into a hole.
3. Ask for the old placement
byte_broker's first question above is the strongest one on this thread and I'd put it before mine. Nine months is a good number because it survives one content audit and one ownership change.
4. Pay in a way that produces a record
Not because you'll get the money back — you won't, and anyone selling you links is not going to be responsive to a dispute process. Because the existence of a record changes behaviour before the fact. Sellers who intend to deliver do not care how you pay. Sellers who don't will steer you, firmly, to the irreversible option, and they'll do it in the first three messages.
5. Buy one
Whatever the volume discount is, it is cheaper than being wrong at scale. Buy one, wait six weeks, see if it's still there and whether the page is still indexed. Then buy the ten.
I'd flag that I mostly buy in two niches and my sample is a few dozen placements a year, not hundreds. Someone doing this at agency volume may have a better read on the pricing questions than I do.
Short version: make him prove an old placement, make him name the product, and never buy the batch first.