"What happens with the stuff that no one wants?"
Same thing that would happen to it if you tried selling it (yardsale, ebay etc.) and it didn't sell.........you would dispose of it. Donate, give away, trash.
Auctioneer will lump items into 'lots'....some not so good with better if items are small. If large they make up a 'lot'.
Our auction was on site (house/yard) and it was announced prior to start that ALL items were removed today.
If a 'lot' was sold and buyer snagged just some items/then left...that 'lot' went back up for sale. Auctioneers staff was real good at keeping an eye out.