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Not if she does so, unaided by you. That act, though seldom used today, is one of the selective enforcement devices used for underaged prostitutes.

US Dept of State signed on to the United Nations definition with:  
http://www.state.gov/j/tip/rls/tiprpt/2008/105487.htm

Meet at least one item in each of the three columns and it's considered trafficking.

Overseas if a provider travels to the UK to be paid for sex (which is a legal occupation) it's considered trafficking.
http://en.wikipedia.org/wiki/Sexual_Offences_Act_2003
However, if the provider emigrates to the UK and changes nationality, suddenly the same act is no longer a crime.  Go figure

Only if you are involved in bringing them across state lines.  What you know, what you see doesn't matter.  What you do does.

How about those guys who send tickets for providers to visit them in another state....Mann Act.   I haven't heard of the feds considering this worth their time, unless there is a large number of people involved.  However, the trend coming from the midwest is to stop demand.  That is prevent the men from seeking prostitutes (good luck, some people never learn from history)  may make this and even very minor infractions to be interterped an act we don't think much off as an element of the prostitution, pandering laws

Human trafficking will be whatever the authorities says it is and you may not even have to have knowledge that you are breaking a law