system
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human
[Back to the Future] How successful could Biff Tannen actually have been betting on sports?

I feel like one of two things would eventually happen: 

* People get suspicious of all the winning bets, so he'll run out of counterparties or piss off the mob or get the Feds sniffing around
* His wagering will eventually affect the outcomes of the games themselves, for example a player involved in an unusual play gets accused of fixing the game and becomes distracted by this, or a team finds out Biff has bet on them and this affects its mindset during the game.

In the first case, if the feds (has to be them, he owns all the lower law enforcement) or mob raid his premises, they'll probably uncover the Almanac. In the latter case, Biff's timeline would become different from the timeline in which the Almanac existed.

Biff doesn't seem clever enough to sprinkle bad bets in or otherwise cover his tracks, so I'm not sure how long he'd actually be able to make money on gambling.

I don't think he can own the mob that already was in place when old Biff went back to 1955. He could start his own mafia I guess and cut some deal with the "legacy" mob, but in the long run I don't see an advantage; they legacy mob would want him gone at some point. They're not worried about a 20-year-old kid with no family connections.

How does he keep it up for 30 years?
gpt
It's not actually that hard to keep it under wraps.  You just have to lose about as many times as you win, which isn't hard to do when you know the results.  
You go about 50/50 at the start, keep it just at or above breaking even, until you're a known gambler.  Then you bet a decent sum on a longshot you know is going to come home and get a big payout.  Nobody's suspicious of that, it happens.  
From there you win some more big ones, but you also lose some big ones.  Keep it in your favor, but throw away more money than folks would think you ought to if the fix was in.  
Get another big break every once in a while, and then just keep going like that until you have your network established and can switch over to other income streams.  
It's something even Biff can wrap his head around, the only thing that might cause him problems is discipline.