I make good money selling links, but I'm careful about it. Basically my goal is to have purchased links be as useful (and as useful LOOKING) as my natural links. That means saying "No" a lot, being strict in my terms of sale, and sending a lot of people onward who wanted to pay shit money for shitty-looking links to shitty places.
Porn sites labor under a lot of SEO adversity already these days. I'm not saying "don't worry about penalties" because it can always get worse. But I am saying, sometimes selling links doesn't have to make it worse. On the other hand, if my SEO on a site was "killing it" I'd be a lot more reluctant to sell links on that site. The price would go way up, and I'd sell a lot fewer.
I don't have any links that are as cheap as $150 bucks. But then, most of the links I sell are surrounded by high-quality unique content, and the only way to guarantee that is to write it myself, which doesn't come cheap.
Last but not least, sign a contract? They can
That's not how it works with me. You send the money, I put up the link. It costs thirty thousand bucks minimum to get in and out of a court with a contract enforcement case these days; if the transaction isn't larger than that, a contract is just a waste of your time and mine. Meanwhile both sides have the emails setting up the deal; that's enough evidence to establish a contract in fact if somebody really does want to sue in small claims court or something stupid like that. LOL, no contracts.