There are so many replies here that I will probably die of age before I can read all of them, so instead of piggy-backing on whatever discussion is happening right now, I'm just going to answer the original question while drunk. I guess the most cringe-worthy line I've ever heard in porn comes from this one scene I watched when I was like fifteen (yeah, I've been watching porn for a while, I don't fucking care what you say, I regret nothing). Long story short, the entire prelude revolved around a girl that couldn't pay for something, and the guy at the cash register or something says, "There is one other way you can pay me. Follow me to the back." In real life, there is nothing so valuable (besides jewelry or some shit about that cost, but then you probably brought the money to pay for that) that you need to spread your legs to leave the store with it. If I were a girl and in a situation where I can't pay for something, I would just not buy the items I can't pay for and move on with my life instead of letting some perv fuck me with a micropenis. Plus, don't you think it is a tiny bit suspicious that a cashier is taking a customer to some employees-only room? I know I would get suspicious if I were a bystander and saw that (to be fair, I might just mind my own fucking business because I have my own problems). All in all, the whole cliché of "sex can be used as cash" is pretty stupid in my opinion. Then again, porn is an industry that thrives off clichés and non-original ideas. Ironically, I find situations where a girl thanks someone who did something like save her life with sex quite arousing. This is pretty different from "sex can be used as cash" since the girl has a stronger reason to want to fuck the guy other than "I want four more boxes of cereal" because he saved her life or something. The original question mentioned the stepfamily fetish, and I honestly think the whole stepfamily sex trope is banal, so I don't really focus on lines there because they are all equally dumb and overused. Now, I'm going to post this reply and wash in cold water to sober up.
Yeah well, before getting into the matter, breaking some paragraphs here and there will help with making your post a little more readable.
I'd like to agree with you on the 'girl paying with sex' trope being stupid, but fact is reality kind of proves you wrong almost daily about that.
Talking this from a writing angle instead, just a few sparse thoughts:
-You are right, it would be silly if a girl paid with sex for a jewel, but think about that, the one for which the whole ordeal wouldn't be worth would probably be the cashier. He would be the one paying sex way more that its current, err, 'market price' for once. He would be the one risking to get fired from a job he probably needs more than a cheap fuck in the back of his shop with an unwillnig partner.
-Out there though, people like that, I'd like to say sexual predators, or better a scavangers, do exists tough. Those kind of men take advantage of women in need, and they have a keen eye to spot them. And they try their luck as many times as they can when they are in a position of advantage, one that has authority or power or both, while still feeling safe.
-And we reach our third point. The question of the
need. I mentioned it already, didn't I? As I said before, generally it would be safe to assume the cashier/clerk or whatever needs his job. What about the girl needing that specific something? You are right, if the girl doesn't need that jewel, she could just walk away and go on with her life. Easy.
But let's just assume she needs it. And let me tell you, you
always assume she needs it in a story, desperately so, I'd like to add, because that's dramatic.
And you want drama in your story. And why would you even mention it if she didn't need it in the first place?
If she needs it, badly, would it be food because she's starving or her mother's jewel that she pawned and now she's lost the money to get it back, then you have your context set up.
This kind of stuff applies to all situations where sex is traded, be it black mail, or concussion or whatever comes to your mind.
There could be also something to say about the girl's status and attitude towards sex, but let's leave it for now.
It's not that stupid if you think about it, it's just all about the right context.
Now though, you can still write this stupidly if you can't set it up properly, I give you that...