I'm a massive fan of charity shops, we pretty much buy everything (bar food!) from them these days. Not necessarily because we can't afford things new, but because I'd rather save, or spend money on other things than buy something new that I could get secondhand for a fraction of the price. There is the environmental issue too, using something again halves the impact of it's manufacture and could prevent it from ending up in landfill. Plus, you are giving money to charity for the item. When we've finished with it we can take it back to a charity shop and they can get more money from it again, and so on.
Mainly though, I'll be honest, I just love a bargain. And all this Duplo for £2 was a bargain!! (yes the set was pretty much complete - all the important bits were there - the digger, the crane, the truck, the house and the people!)
Kit loves it so much, I went to check on him after I'd put him to bed and he was still playing with it!