I made a tidy sum on matched betting some years ago, basically a guaranteed way of making money on such sites if you know what you are doing. And yes, it's entirely legal (if morally dubious blah blah blah), at least where I am. However, the betting sites will likely catch onto what you are doing eventually and clip your wings by restricting your stakes or banning you outright.
https://en.wikipedia.org/wiki/Matched_bettingI've also dabbled in betting exchange trading. Again, it's entirely legal. Basically it works in a similar way to the stock market. Far more difficult than matched betting though and I've never really made money in it.
Rarely used them for what would be considered actual gambling.
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