I played professional poker for several years up to
Black friday.
Thus I can attest from my own experience that it's very possible to make a very healthy living just gambling.
However I only played poker, a game in which I had a clear edge against most of my opponents.
I did play other casino games like roulette, but merely for pleasure. Any claims in this topic of consistently beating regular casino games should be taken with a good grain of salt, as they are definite -EV games. But they can definitely be fun
You probably also remember casino bonuses, if you exploited them, or as the term was, "bonus whoring." When Internet gambling was a novelty (and to some extent to this day), there were often lucrative deposit bonuses which you'd get and then be able to turn into real money by giving a certain amount of action or "playthrough." Like for 20x playthrough with a $1,000 bonus, you'd have to give the house $20,000 in betting action. So any game with a house edge lower than 5% is an expected profit. However, many casinos have stopped being so generous with these offers, or specifically exclude the low house edge games like blackjack, video poker, etc. from deposit bonuses. Also, they tend to kick you off for "abusing" the bonus (by making money on it). I never had any of them refuse to pay on a bonus or confiscate funds, though, even when I got kicked off all the Cryptologic casinos for basically raping the shit out of their bonuses. This casino network with a lot of sites in it gave a monthly small bonus for a certain amount of play, and one month, I collected it on every site betting the entire playthrough value, something like $500 or so, on a single hand of a 21 variant called Pontoon. Basically won 10 out of 12 hands and that was enough to wear out their patience.
There's also arbitrage betting, where you find two sportsbooks offering odds such that whatever outcome the sports event has, you still are going to make money even taking out the house vig on both bets. For some reason, sportsbooks seem a lot faster to kick you off for advantage play, though, and then arbitrage opportunities dry up. I have also found that in many of these arbitrage situations, one of the sportsbooks majorly screwed up and one of the bets is actually massively +EV. But since I generally don't know jack about sports, I stuck with the sure things here, usually.
Casino tournaments are also a very lucrative casino opportunity. In these, which are often partly skill-based games like blackjack but sometimes pure luck games like slots, you buy a certain amount of tournament chips (with no independent value), sort of like in a poker tournament, and it pays off in real money based on how many chips you win in the game play. This can be winner-take-all or have a pyramid type payout structure with a big first prize and smaller prizes for second place and on. Since most of the people playing these, other than maybe the blackjack players, are usually just recreational gamblers, it is possible to have an enormous advantage over the field.
Progressive jackpots are also often a source of +EV bets. Basically, on every bet on an otherwise losing game like a slot machine, the house takes a drop that then goes into a jackpot that increases in size over time. So at literally no risk to the house, they get to offer a jackpot that when it gets huge creates a great deal of excitement. Since these are usually on extremely unlikely events like poker bad beats or some goofy 5 of a kind on a slot machine, these are rarely exploitable by a single player with a normal size bankroll and require some kind of team play to make a regular practice of exploiting.
Anyway, I've never found enough of these kind of opportunities to be a reliable source of income by itself, since other than bonuses, they're generally occasional events, but they can make a nice change of pace from poker from time to time.
There are also, of course, rare occasions when a casino accidentally offers action that is favorable to the player. Obviously, nobody is going to tell you about those if they know them.