As may said above, I would personally never recommend gambling in any online game. Alot of high profile content creators have been given increased odds or given a inventory value worth thousands to increase their chances of winning. The house always wins. This also goes for betting on ESports games, now while this has become more of a regulated industry, it's still very risky as with all gambling. Match Fixing will always be a thing in all professional sports.
Certain MMO's allow you to sell in game items / characters on third party markets to make a quick buck, you should read and look up the ToS for these games to see if they allow it. Other wise you'll grind for hours and just lose your account the minute you sell / buy these items. *Cough* Fifa Coins *Cough*
If you want to make a bit of quick cash online I would recommend learning a skill / trade and setting yourself up on Fiverr and other Freelancing sites. I personally use some of my skills as an Auditor to do the odd booking keeping work for people online, it's not alot but for what I do it's pretty easy work.
Other options also include day trading such has Forex trading, futures, options & the like. This is something that requires a bit of money to get off the ground & a good understanding of the markets / global economy. If you are considering this, I would highly recommend reading upon about this. This isn't a guaranteed way of earning money, as with everything it has it's risks.
Hopefully this helps.