Not sure what the difference is, but I just bought a NIB MG3520 for $39.99 and free shipping:
http://www.buydig.com/shop/product.aspx?sku=CNMG3520K
So far it’s working great!
After checking out Buydig.com I would say the difference is in whether you have faith/trust in the seller:
There is a class action lawsuit pending against Canon for their MG series printers print-heads becoming faulty for no reason.
Happened to me after ~ 6 months.
This one isn’t on the list of affected printers.