Both previous interpretations are correct, but I'll elaborate a little more.
At least in the American vernacular, something "up for grabs" means something that "hasn't yet been assigned/attributed to an owner." The phase also has a connotation that the somethings will be assigned/attributed to an owner very soon. [Edit: Also, something "up for grabs" is usually chosen by its soon-to-be owner.]
As an example, suppose you and your five family members are drawing numbers to determine who has to clean the house. You write numbers, 1 though 6, on separate pieces of paper, and put those pieces of paper into a hat. Each family member (including yourself) draws a number. Eventually somebody rolls a six-sided die, and whoever picked that number has to clean the house. Suppose the twins pick their numbers out of the hat first, and immediately after they pick, your mother asks, "who picked 4 and 6?" After looking at each-other's numbers, the twins answer, "they're still up for grabs," meaning numbers 4 and 6 haven't been chosen yet and are still in the hat.
As another example, suppose that during an electrical blackout, there's a run for candles at the grocery store. You notice your friend in the checkout line and you ask her if she was able to get candles. She replies, "Yes, fortunately. And there should still be a few up for grabs if you hurry." She means that if you go to the isle that has the candles, there should still be a few left, assuming they haven't been taken already.
I believe what Carl Sagan is saying, is that if we are not able to ask skeptical questions and allow ourselves to be skeptical of those in authority then we are essentially allowing ourselves to be swindled by the next charlatan* that comes along.
*(Or whoever comes along and wants us to believe whatever they want us to believe).
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