I've exchanged sex for various favors in the past, but there was already a mutual desire to get it on. She enjoyed being taken advantage of, and I wasn't one to turn her down - readily obliging and having a very nice time of it.
There's nothing wrong with using sex to obtain what one wishes, assuming you are not deceiving the other person into believing that your intentions are out of love. Don't lead them on if you're a decent human being, because you'll eventually have to let them down.
When in a loving relationship, I see no reason not to tease eachother and fool around in a way that resembles currency exchange. You know it's for play only, and hey, if you really want that dress (and he really wants that blowjob), go ahead and earn it.
