If prostitution is legal in any particular country and the assumption that a contract is made - sex for money - and where both participants are fully aware of this and adults, then I see few issues. As long as those providing the service have not been coerced into such, are of legal age, and this is their free choice to do so. Simply because of the notion of free will and autonomy. But they should have adequate knowledge as to what this will entail when making such a choice.
Many sex-workers will likely travel to other countries simply because they will earn more than in their native country. The numbers who have actually been sex-trafficked might be hard to obtain, given that they might say whatever depending upon the circumstances. Those who might have been drawn or coerced into sex-work is a more difficult problem, especially if this started when they were underage. In this latter case then they almost certainly will have been exploited and hence could be seen to not have acted freely - thus invalidating any free choice argument. And of course if many are coerced as adults as to such then this too should be seen as being exploitative.
But I should add, resources should be available to discourage any from taking up such work or as to supporting any who do.