There are certain quests that require stuff to be sold through auction (one growing potatoes and tomatoes and auctioning them), so some sell them cheap just to get the quest done fast. Other times people may not be aware of the sell to vendors feature or like Djoe said, maybe just people being nice :)
Really? Which auctions are you looking at? The ones I tend to see have items on auction for *more* than what a vendor sells them for e.g. butterfly egg for 499 when the vendor sells them for 400. I mean, is anyone really that lazy that they would rather overpay than hop a couple of streets to the vendor?
@Joojoo -- Yes. I am that lazy. :3 At least, when I have the spare cash. Also, it's nice to support your fellow Glitchen by buying on auction rather just going for the vendors.