I buy and sell stuff occasionally at auction. I used to buy the odd storage unit, and pick at garage sales, so i would end up with lots of random stuff to sell.
I found for the auction place i deal with, these are the things that were not worth sending to auction
* low-value large items because of the generally low return on the time and cost of transporting them there vs listing on craigslist and having someone else pick them up
* valuable items because they are usually easy to sell on craigslist and not lose a 1/3 or more of the value paying commission. Also, for me it's hard to stomach the risk of something worth $1k selling for $50
* garage-sale type household stuff (VHS/DVDs, Kitchen stuff, books, clothes etc) because they don't really sell at all, or for virtually nothing. Thats what garage sales are for, LOL
Instead, what i've found is there is a sweet spot for small, relatively cheap items with good re-sale value, but aren't worth listing on craigslist, and are also a pain to try to sell at a garage sale because everyone only wants to pay like 25cents when its worth $5-$10.
* Semi-collectable coins, sets and paper money (do thorough research first on these, though)
* 70's, 80's and newer comic books and sports cards with not much value on their own, but in small lots they do ok
* Any "guy stuff" Tools, fishing and hunting gear, man-cave items and that kind of thing.
* Other inexpensive small collectables, art, etc
Just know that for auction places, each auction night can different, so as with investing, i dollar-cost-average selling items at auction. For example, if I wanted to sell the following items - a box of 100 comics, 12 coin sets, 4 signed art prints, 2 big boxes of hand tools, and 6 fishing rods. I wouldn't send them all to auction at once for the same sale. And i also wouldn't send them in Week 1: All the comics in a big box, Week 2: All 12 coin sets... etc.
I find when buyers see something like a big a big box of comics for example, they don't VALUE it as much as 20 small lots of 5 comics over many weeks, I think it helps to provide the buyers with a false sense of rarity by doing that, so this is how i would send those items in
Week 1: 20 comics in 4 lots, 3 coin sets, 1 signed print, a half box selection of hand tools, and a pair fishing rods
Week 2: 20 more comics, 3 coins sets, 1 signed print, another half box of tools, and 2 more fishing rods
Week 3 and 4 repeat, repeat, etc etc... until everything is sold over 4 or 5 weeks.
From each auction some things will sell for what you expect, some things will sell for WAY under value, and some things will sell for WAY over value. Breaking up the items into a variety of small lots over time helps smooth out the bumps, like averaging in and out of the market does.