How guest post reselling actually works
Most explanations of this business are written by people who have never done it. They describe a marketplace. What actually exists is a chain of private arrangements, and understanding where you sit in that chain is most of the job.
The chain
There are four positions, and one person can hold more than one.
The publisher owns a website and will place an article on it for money. They are not running a media business with an advertising department. Usually it is one person with a WordPress site, a Gmail address and a WhatsApp number, who answers messages when they feel like it.
The supplier has a relationship with a few hundred publishers and sells access to them as a price list. Their sheet arrives on WhatsApp as a Google Sheets link with a banner across the top and the actual headers three rows down.
The reseller — most likely you — buys from suppliers or direct from publishers, and sells to people who need links. You hold no inventory in any real sense. What you hold is a list of prices you have been quoted and a rough memory of who replies.
The buyer is a business that wants links pointing at its own site. An agency, an in-house SEO, sometimes another reseller.
Money moves down the chain and links move up it. Each step adds a margin, and each step is a private negotiation with no published price.
Why the same site costs different amounts
A publisher quotes what they think the person asking will pay. That is not dishonesty; it is the only pricing information they have. A reseller who has sent them twelve articles gets one number. Someone writing for the first time, from a domain the publisher has never heard of, gets another — usually higher.
This is why a shared price database would be worse than useless. Two people can hold genuinely different prices for the same site, both correct, and neither is entitled to the other's.
It is also why the cheapest source for a domain changes. A supplier who quoted $60 last quarter may have lost that publisher entirely, and the version of their sheet you are working from will not say so.
Where the margin actually is
The naive sum is what you sold it for minus what the publisher charged. That number is wrong, and it is wrong in the same direction every time.
Around four out of five placements require you to supply the article as well. Someone writes it. Whether that is a freelancer at $20, a writer on retainer, or your own evening, it is a real cost against that placement. Leave it out and every deal looks about $25 better than it was.
On a placement sold at $150 with a $70 publisher cost, the naive margin is $80 and the real one is nearer $55. Across a hundred placements that gap is the difference between a business and a hobby.
So the sum worth tracking is: sale price, minus publisher cost, minus content cost. Any markup rule you apply should apply to both costs together, never to the publisher's price alone.
What goes wrong
Prices go stale. A quote you were given four months ago is not a price. If you quote a client from it and the publisher has since doubled, the difference comes out of your margin, because you have already committed.
Links disappear. A publisher redesigns, drops a category, sells the domain, or simply removes the post. Nobody tells you. The buyer finds out, sometimes months later, and by then the supplier's guarantee may have run out.
Follow-ups don't get sent. Most publishers do not reply to the first message. They reply to the second or the third. Almost everyone who does this work keeps a spreadsheet for it, and almost everyone lets that spreadsheet rot.
None of these are hard problems. They are all the same problem: the state of this business lives in a chat history, and a chat history cannot remind you of anything.
What this means for tooling
The useful tool here is not a marketplace. It is a record of arrangements you already have: what each site costs you, from whom, when that price expires, what you sold it for, and whether the link is still there.
Everything else follows from that record being accurate.