It doesn't seem to be possible to auction bags, yet these are evidently auctionable items, as if you type 'bag' into the auction search, it tells you that there are no open auctions for them. If you click on a bag in the 'New Auction' page, it opens the bag, rather than placing it for sale - if it's empty, then I'd expect the bag to put up for sale.
Perhaps a related issue, is that bags cannot be sold to street spirits either; dragging the bag into the relevant area does nothing - no error message, it just doesn't work.