First of all, the people or institutions or whatever he's buying his carbon credits from are, I presume, all po' folks who couldn't afford Gore's greedy, wasteful lifestyle anyway. I don't see any net reduction in their carbon emissions to offset Gore's flatulent lifestyle. They were going to live the way they live, and Gore was going to live the way he lives; he's just paying them to be good for him. Sort of like the old Scots "sin-eaters," who would take on the sins of a dead man at the funeral, in exchange for some grub.
Perhaps a more politically meaningful analogy would be with the Civil War military draft. The Union needed a whole lot of soldiers. So they instituted a draft. But since the number of warm bodies was more important than exactly whose bodies they were -- and since the Union also needed money every bit as much as it needed men -- rich people were allowed to purchase the services of a volunteer to take their (or their sons') places. Which meant that poor people fought and died, while rich people "did their part" by putting up some cash for the war effort. Nobody saw this as unfair back then, though we would see it so today.
In any case, it's not really possible for a sin-eater to assume the responsibility for someone else's sins, nor can one say (in today's political world) that one has "done one's part" by paying someone else to do one's civic duty for one. If it is really incumbent upon us all to reduce our consumption of the earth's treasures and our production of the toxins that threaten the earth, then there is no exemption for rich people. Everyone must do one's part, and Gore is just another rich blowhard who wants to tell everyone else what to do while remaining free to enjoy his own bloat.