The question is not "which hurts developers more". They are two completely different things.
Piracy is illegal and developers have every right to pursue it and to get upset about it.
Buying used games is your right as a consumer. EVERY industry has to deal with used copies. You can buy used cars, kitchens, houses, TV's, PC's, etc. But I don't see people from other industry bitching about people buying / selling used stuff.
Selling your games used is your right as a customer. I can't believe how often piracy and second hand sales are used in the same sentence, often like they are equally "bad" or anything. Piracy = bad, selling / buying used games = freedom guaranteed by our most basic rights.