My grandfather was an account for horse racing bookies in London (this is back in the '50s). He once noted to the owner how surprised he was that they made sure to pay out so fast. It seemed so honorable, yet the business had such a shady reputation. The owner said it's so important to pay up fast, because that will ensure the winner comes back again for more. The house will always win.
There will be plenty who will be careful, follow the instructions correctly, and pocket some nice change. But there will be a few readers - not "there may be", there will absolutely, positively, 100% - who will either mess up on the instructions and lose their money, or worse, will get their feet wet and then ride that high of "winning" to placing another small bet, then another, and another, etc..
Please be careful.