Basically, the difference between sex work and trafficking is that one implies consent and the other does not. Sex workers go to a great deal of care in negotiating with clients the terms of the transaction to reach consent. They negotiate over things like where they feel safe being taken, what acts they're willing to perform, condom use and other safer sex practices.
Human trafficking victims do not have the option to negotiate at all. That is all done for them, and they are forced into their circumstances. There's quite a difference between sex work and trafficking.