In regards to it being standard practice to nego on surplus loads. Liquidators will have a set price on loads, but that doesn't mean that you can't at least try and negotiate with them.
Of course liquidators work on a broad based wholesale business model, and the absolute best way to negotiate the best price is to buy more, but you never know if you don't ask!
In regards to clearing stock, such as your shoes. You can always look at bundling products. Instead of asking below on one pair, offer two pair at cost or with a small margin. This offers the buyer good value on their purchase, and saves you from taking a hit.
The one thing I would also suggest in terms of what isn't selling, take a close look at why it isn't selling to start with. You'll find just because something isn't selling, doesn't automatically make it unsellable.
I'll use the story of the knife set missing most of the knives. One person will look at that and think, well that's pretty much worthless because it's missing most of the blades. Another will look at it and say, well I have a couple of products to sell here.
They will then sell what knifes there are, and the knife block itself as separate items.
Perhaps it's a late model vacuum cleaner that isn't working, that doesn't mean that there isn't a market for some of the parts that you can break down and sell.
I've dealt with a stack of second hand stock over the years, and rarely have I found that I have been stuck with anything that I couldn't make money on in one form or another.
I hate to go all Zen on you here lol, but treat a product like a room full of windows. If one window is closed, move to the next and so on, sooner or later the chances are that you'll find one open :)