She's literally not doing that. If you reread the series, like chapter 16, you will see Ars didn't even want to start doing the whole coaching thing because there was nothing in it for him. As the coach, he would have obviously already known it will involve sex, but he still was going to refuse because it's not worth it for him. The only reason he reversed his refusal was because Rua told him that the school curriculum includes teamwork, so his individual strength might not save him if the rest of his group sucks. On the other hand, he was afraid Rua would spread the news of his "part-time" job she happened to witness at the casino. So, the only trade and benefit, for which anyone is nominally doing this, is to make Rua (plus Rena, it should have been) stronger, which would also benefit Ars, supposedly, as well as keep Rua silent about his secrets.
If you reread this same chapter, you will see Rua actually apologised to Ars for making him go through this time after time. Since we can assume he has been doing this with the assassin group lady since he was a little brat, he's no virgin desperate for sex with any girl.
So, in no way or form does she sell her body for anything. It does look like she enjoys this, though. Hard to say about Ars with his dead fish expression through it all.