PSA if you have a story with at least a few thousand reads, you might recieve a private message asking you to move your books onto their platform - for profit. Be wary of anything like this. If you write fanfiction, know it is illegal to make money off of someone else's intellectual property. Chances are you will be ignored, but if you're actually successful selling a fanfiction, there is a possibility you could be taken to court for it. Most countries have agreements to enforce one another's copyright laws as well, so where you live likely does not exempt you from this. Do not be drawn into something you didn't know was illegal.
If the book they want to buy is an original, still be careful. What they're doing may be legal, but it is against the Wattpad terms of service. Don't make any deals without fully knowing what you're getting into.