>well, in the case of sex-trade it can be argued that the purchaser is ,generally speaking, in a better state-of-living than the seller.

Maybe this applies to homeless street walkers. But the majority of prostitution I'm aware of (anecdotal, I know) is middle-class men who go to freelance call girls 10x wealthier than they are. The customer pays a several days' worth of his wages for 2h of fun. So I would say it can be argued that the purchaser is a bigger victim than the seller, considering the financial power dynamics.