I don't use addons, but even I am aware that people who do use them will sometimes be able to work faster then me.
It's not like they're botting or anything, there's still interaction involved in the purchase.
And then there's the golden rule of the Auction House: "If you missprice stuff, it's your loss, deal with it"
It's not like this guy I once made a deal with to buy something COD, and he forgot to check the COD (so he actually sent me money
In those cases I do the honorable thing (after all, like in any businessdeal, a word should be as good as a signature) and I send back both money and payment.
But ask a cheap price on the AH (neutral or faction) and It's "finders keepers", no point in going all cry-baby