I am of the opinion that no one can win betting one way for a lifetime. Just my opinion, but I have yet to see anyone that has gotten super rich doing it.
You can make a living getting favorable lines and hedging on and off positions.
I did it for about 7-8 years and made more than enough to live on.
Starting bankroll is relative. But a mid 5 figure is probably realistic unless you live at home and don't have any expenses.
As for selling picks. Not a great way to make money either. Basically you need to be a good enough salesman to sell them, and also a good enough picker to at least fool people into thinking you can pick enoug winners long term to help them.
Most of the angles and areas that are most profitable are usually available for only a few minutes if that, simply because the advantage players are on them so much quicker.
It takes a lot to do it, and even guys that are "smart' enough to do it, might even fail. You need a certain mentality and demeanor to do it and have optimal results long term.
EVERYONE can win when they are winning. But throw a few losers in there, or a real bad streak, and you will see where the people that can do it and can't do it are seperated.
If I had a dollar for every tout or poster that claimed to be professional and then came in here and cried about how they have lost everything and quitting, due to a bad run, I would be rich as well.
Best advise I can give anyone. Don't even START gambling, it will make it a lot easier on you in the long run.