I'm not an expert, but most of what I've bought on Amazon (mainly books, CDs and DVDs along with a handful of other stuff) seems to be cheaper on Amazon.
I don't see anything ethically wrong with buying something at a 50% discount and selling it at retail on Amazon. I would consider the shipping costs and the risk of getting stuck with leftover stock (especially if its an item that may go out of style).
If the shipping is reasonable, I'd buy a few to sell to test things out (companies shipping time, costs, any unforeseen issues, etc.), then go from there.