Hmmmm trying to think of a good way to explain it tbh, explaining slang can be quite hard sometimes!!
Taking the mickey is just another way of saying making a joke, but it’s generally used when the joke is at someone’s expense. The connotations of ‘mickey taking’ can be from light hearted banter through to being really quite rude and mean about someone.
So back to wind up merchant, merchant being a trade, so it implies someone would be doing it all the time, day in day out. They would be winding people up by taking the mickey all the time, again could be either light hearted or quite mean, could extend on the internet to trolling which I think is the inference of the person who posted it.
Phew…… that make sense?!