Sometimes. Often the sure fire bets have lower returns than the expected annual stock market returns.

You have to watch out for resolutions are don’t depend on the truth or could be abused.

Examples of markets to avoid are those that a single individual can manipulate. They could take the most profitable side and corrupt the result.

Presumably it's a negative expected value game - the company running it has to make money after all... so likely not.

Not sure what you mean exactly. If you mean the virtual rates, a gambler has no way to make money by placing bets there (while the real probability is 1/8, the odds the betting company is selling are of course much worse). If you mean in a broader context that it is easy to take these people's money, yes and no - there is a long queue of people wanting to do the same and what they want to do is mostly either heavily regulated or illegal.