^ I've always felt that the difference between scalpers and resellers is a matter of timing. Scalpers buy up high demand or reduced price items to flip for short term profit. Resellers buy up items to sell for higher profit over the long term.
I was reading this thread yesterday (I have two young children, so it piqued my interest), then today found a complete and boxed #5653 Stone Quarry in a charity shop for £5.99. The box and instructions have seen better days, but the pieces are great. There were even a few extra bricks in the box and a handful of compatible Mega Blocks.
I think the reason that Duplo can be found so cheaply is that there are far fewer AFODs (is it even a thing?) than AFOLs, so it's not hoarded as much and it's often cleared out as kids grow up. I often see it at car boot sales whereas Lego is much harder to come across.
Well, today must be my lucky day! Saw a couple of ads on Facebook tonight (same seller) with very blurred photos, one of about 30 minifigs (I recognised a few CMFs despite the pictures); one with a pink box of parts; they were asking £5 for the figs and £10 for the box. I offered to buy the figs, then when I got there had a look in the box and offered £13 for both lots. Upon excited inspection at home I found #5560 (the pink box), a handful of polybag sets (2 Harry Potter, one Batman, one Star Wars, one Cars and one Friends), a Team GB Horseback Rider CMF and an almost complete set of series 5 CMFs (with some duplicates). All that is missing are the Gladiator's head and helmet and the Lumberjack (though his axe is there).