Come to think about it, rebuying last 10 (or so) sold items on regular stores prolly would useful too.
I too have suffered this fate - the over zeallus pressing of the zero key can prove costly 
The 'No to Sell' slider defaults to 2 iirc but if you have 8 of an item to sell then set it to 8 and sell your items in descending ofer of number carried (8 Luck Charms, 7Boresight, 7 Kinetic Weapons, 6 Rubies etc) as this way it will stay at the level you need.
Golden-Phoenix - Lvl 50 Fire/Fire Tank
Oodja Nikabolokov - Lvl 50 SS/WP Brute
Baby-Phoenix - Lvl 50 Peacebringer
How much wood would a woodchuck chuck if a woodchuck could Chuck Norris?
Maybe have a warning window come up if your bid is twice as high as the latest sold price?
@Golden Girl
City of Heroes comics and artwork
Actually the number to sell will default to the number you have (or 10 if less) of the first multiple item you put up
Mind of Gaia lvl 50 Defiant's first Mind/Storm 'troller.
Deadly Doc 50 Dark/Dark Corr
and lots more on Pinnacle,Union and Defiant
might be me but uhhh, i can see the sell price so.....i can sell and buy for a good price...without any tool.
i think it's more about how well you look before you act, if you rush then you lose more then needed.
But we are all human. We make mistakes. A system that prevents us from making stupid mistakes is good.
I approve of this warning pop-up thingy! 
- @DSorrow - alts on Union and Freedom mostly -
Currently playing as Castigation on Freedom
My Katana/Inv Guide
Anyone who doesn't take truth seriously in small matters cannot be trusted in large ones either. -Einstein
I talked around with people, since im fairly new (6 months) to the game, about accidently bidding to high on the auction (Wenthsworth/Black Market) and got the impression its not that unusual (thank god, im not alone).
Frankly, im a bit surprised that this issue hasnt been "corrected" already.
My suggestion would be an auction "tool" (as part of the WW/BM), where u can set a max bid nr. and if your bid exceedes that set nr, a warning sign would pop up and a warning sound (u should be able to turn off/on the sound as part of the tool as well).
Also, when putting duplicate items on sale, auto set the sell nr, so u dont have to rewrite it every time.
Peace out!