I disagree. Someone who has nothing to lose has everything to gain. Someone with nothing to gain has everything to lose. Assuming that these two positions apply to everyone, or something in the middle of the two apply to everyone, it would be an absolute statement. I think the people that can't be bought, monetarily or otherwise, are either those with a balanced perspective or those who just quite caring.
For instance; a parent who doesn't care about their child, wouldn't do anything to harm them or to save them, they could remain neutral. Or someone who is suicidal and values nothing in life, be it because of depression, fear, et al... Wouldn't be easily bought because they've already given up.
Another example would be something like people who subscribe to reincarnation or an ideal of an afterlife that holds them morally accountable for things done in this lifetime. They wouldn't be easily bought if something went against their belief system (and this topic denotes going against ones principles, as a person who has a neutral stance wouldn't be being bought, rather paid).