What normally happens in such situations? Is re-auctioning another possibility? I've always imagined that not all domain names that are put up on auction end up being bought. Does this lead to a drop in the buying price?
It depends on where you run the auction. Re-auctioning is always a possibility and some places like eBay sometimes give you a second auction at a discount. If it hasn't sold at the first auction you are probably better asking a lower price for it.
Not necessarily, but it sure would lead to a drop in the next starting asking price when the owner puts it back up for auction. At least it should if the owner wants to sell.