Even though most of the sellers won't bring any good (if any) results, paying based on results is not ideal for sellers,
Sellers would need to make sure the overall website (on-page) is in point, which is not part of the deal, should be billed separately,
As well as niche/KW research, so they make sure their links would be enough to make you rank, that's again not part of the deal,
Results take time, up to 3 months for the full effect, not everyone willing to wait for that time to get paid for their work, and neither it's fair,
And lastly, define "results" here, is going from the 5th to the 4th page considered results? any type of improvements? that will depend on the buyer expectations, and often, especially for newbies and non-SEOers, it's damn high, wanting to dominate the niche with a tiny budget,
That could work if you're hiring an SEOer to take care of everything, basically as a webmaster, where SEO is the main traffic source,
You have to pay for everything done & all of the work mentioned above, and have a margin of the delivered results set as guarantees,
But, at this point, why should anyone work for you? he's doing all of the work, and if he knows what's he's doing, he most likely has the budget to make the website and compete on his own, which many do, the only thing holding honest/genuine link sellers from doing so is that mentioned work