First, how does one become a pro poker player?
Second, does it work like a sport where you get paid from sponsorships, or do you just directly take home what you win? Or a combination of both?
Third, is this something that you can do part-time, or does it require full time attention?
Fourth, why did you quit?
Note that professional in this context just means made a living through poker. There are a ton of people like myself that earned a good living playing online or in casinos. A small minority received long-term sponsorships.
1. I started playing in high school and college. Like everything else I do I dove in to get better and eventually I was good enough that I was making good money playing online. At that point I started to pursue things full time.
2. For me it was the latter. The very top players in skill and visibility are typically the ones getting sponsorships. Win a big tournament, win at the highest level of online games, or make a tv appearance at a final table and you'll find opportunities for sponsorships. For most players, these sponsorships really just add stability to their income with the bulk still coming from winnings. Though there are the poker personalities who make the bulk of their income from sponsorships and TV deals.
3. You can definitely do it part time at some levels, but to keep up it does require quite a bit of study time to win at meaningful stakes.
4. When the US legal landscape changed in 2011, getting your money out of the sites that were still willing to serve Americans got more challenging. Moving out of the country wasn't an option for me. With the game getting harder all the time due to the proliferation of good strategy material and botting, it seemed like it was time to move on.
So with the online games, etc. is the point just that you are better than the next guy to join the game so you just end up winning against players that are not as good? How reliable is something like that? It's true that a sucker is born every minute, but is it really feasible to make a living off it vs just staying income-neutral (e.g. win some, lose some, not really make any profit)?
Because of this, players of all skill levels will play a long time in games where they are not favored. This is part of why it's so important to keep studying both your play and that of your opponents. If you rest on your laurels, those that do study will surpass you and you'll find yourself playing at a disadvantage without realizing it.
Online you're really able to make a good amount off even a small edge, as measured in big blinds per 100 hands, because hands are dealt so quickly and you can play more than one table at a time. For most of my career I was playing ~1000 hands/hour. In reality due to the rake, the cut the site takes from each pot, you need to have a substantial edge to make money, but this isn't as hard to achieve as you might think.
The main goal of any poker game is to win money. You win money from your opponents, you don't play agains the house. Although , the house (poker site) does take a small percentage of every pot you win, this is called rake. Do note, that in some games, rake can be so high that it is not profitable to play the game even if you are much better than your opponents. The only way to win money on the long run in poker is to be better than your opponents, so you need to look for bad opponents, it is a well know strategy called table selection or "bum hunting" :)
Nowdays it is still very feasible to make a living off poker, but you will have better chance finding bad player in live games.
1. Lots of means this can happen. "Professional poker player" is generally taken to mean "derives primary source of income from playing poker" or sometimes "spends the majority of their time playing poker". There's no exact qualification.
2. Not generally sponsorship driven, although there are some modes of sponsorship that do factor in in some ways - the primary means of income is basically by winning money from other players. A secondary component is often rakeback, in online play. In cash games, you can join and leave at any time and your winnings or losses are simply the amount you are up or down in that particular session. In tourmanent play there is a payout structure based on your placement in the tournament (often something like 25% of the total prize pool to 1st, 15% to second, etc.)
3. It can be done part time. There is nothing to say that your bankroll can't be seeded or supplemented by external means, and you can play cash games for short or long periods. Tournaments are typically long (at least if you remain in them a long time, though you can be eliminated at any time, basically). I used to play approximately 40 hours a week (since I was treating it like a job), but now I play 10-15 and it represents about 30-40% of my yearly income.
4. I personally quit because I found the stress associated with having it as the sole means of providing myself too overwhelming. One can have a stretch of negative earnings that can last hours, days, weeks, even months - and it can be psychologically damaging in some ways. I also found that I preferred to keep it as a hobby than a means of income, since I enjoyed it more that way.