I guess my answer is... you are the parent, he's only 5, it shouldn't be his choice to sit in a booster unless YOU make the choice. :)
Ask yourself if he's mature enough... is he over 40lbs? Is he over 40" tall? Can he sit upright the entire car ride without bending, leaning over, putting the belt behind him, or sleeping? Does the booster (should be a HIGH BACK!) fit him properly... meaning does the shoulder belt lay across his shoulder and chest properly? Does the lap belt lay low on his hips and not up on his belly? Do his knees bend at the edge of the booster?
If you answered no to even one of these questions he should still be in the Marathon.
To answer your question, its a 2007 so it still has 4yrs of life left on it. See if a friend wants to buy it from you, but don't ask more than $50 for it. A friend or family member that trusts you that the seat has never been misused, abused, dropped, in an accident, harnss straps never been washed, etc. Otherwise donating is an excellent idea, someone gets a much needed seat and you get to claim it on your taxes.