Why did Bernie sellout his followers? Did he get another mansion on the lake or something? Did Shill successfully buy him off?

He didn't want to be included in the death toll.

Bernie knew he couldn't win against the Fix the DNC out into place. He supports Hillary because She will appoint Supreme Court Justices who will support her agenda of shifting the USA toward a Socialist Nation that Dominated its citizens.
Bernie Loves that. Liberty for Responsible citizens will be eliminated and the Demon-rats will dictate your future and that of your children and grandchildren.
If Bernie is denied the Power to do that himself he will support the same trend when ad where he can.

He's a socialist;
He'd sell his mother if it meant he'd have some authority within the Democrat Party.

Morals and honesty are never a part of the Progressive/Socialist DNA

Bernie got enough contributions from naive, unsophisticated youngsters looking for socialist freebies that Bernie was able to pay off his overdue light bill. That was all he really cared about.

He conceded despite the cheating that DWS perpetrated in the Dem party. He is a smart man, he knows if he couldn't be president, then changing the landscape would be the next best thing. He wasn't bought off.

running for office is can be very profitable,

esp Pres.

x

And Huckabee, another charlatan, both religious & political, is profiteering from the political process:
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http://tinyurl.com/ochx7s6 . NyTimes 2012
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Altho “Huckabee lost the nominating contest,
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“he was signed by Fox News as a commentator after he dropped out.
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“Since then he has published a best-selling book and
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“now hosts a weekend television program and
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“a nationally syndicated radio show.”
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http://tinyurl.com/p82eovm . CNN May 2015
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Huckabee expects to raise at least $60 million in political donations.
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“Huckabee raised about $16 million in 2008, according to the Center for Responsive Politics, before dropping out of the race in March.”
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“This time around Huckabee expects to raise about $60 million
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“between his campaign and his super PAC in the early stages of his campaign, an adviser said.”
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==
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That was before Trump disrupted his plans.
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x

I think he is actually using all his power to avoid Trump as the president; i don't think he is a sellout - he wants the best for the American people, but was suppressed by his party.