No that isn't why. Other websites/brookers sell it so higher because there giving you a secure, trusted, transaction. They've been around for years selling items, they've become trusted with their rating, and your pretty much 100% sure that you wont be getting ripped off or have any trouble when buying from them. Thats where there extra charge for gold comes through.
Now on stratics you take a chance that you might be dealing with some jackass trying to screw you over. Most of the people are not jackasses but you occasionally find that newbie trying to get a quick scamm goen. Gold is so low yes becuase there were dupes in the past and it has totally screwed the economy up with duped event items, runics, resources etc... None of the gold is duped that I know of. Most of the gold people have are people buying there valorite hammers, event items, etc.. then them reselling the gold, and 90% of the people that did dupe dont sell on stratics... They sell there items for low to other people those people sell the items for gold, and then resell the gold to people for a small profit.