I doubt Amazon would use expensive space in the warehouse to waste value by "letting a box sit in a corner" or something like that. All space in the warehouse must be dedicated to adding value to the customer, not taking it away, or else you're spending money on making your company worse with bad process. What is really going on is that they are actually using the shipping capacity of the warehouse or are getting better at stocking inventory that moves, so there is less idle time to catch up with low priority orders.

I order used books off Amazon constantly, and it's probably my #1 expense after food and shelter. Never had a single problem, but I pay attention to the 3rd party seller's history and read the extended descriptions that most of them fill out. I have no problem ordering from new sellers either and never ran into an issue, exercising due caution is much easier on Amazon than eBay.

I don't understand your argument about price. You seem to be arguing for less choice and consumer power.