Sounds fishy and sounds like a scam. You will need somebody who can vouch. Maybe offer free service for high ranked member here on forum so we know if this works/not work. Honestly I am very skeptic when I hear ads like this offering fast money schemes.
My guess would be that this is matched betting of some sort which is indeed tax free and fully legit.
I know a few people who've done it. It was very time consuming but was dependable. I thought the betting companies were less inclined to foster it these days but there must be loopholes lurking still which is what this poster may be offering.
I could be totally wrong of course.