I would support this.
People already -1 isk things, this just makes it less annoying for dumping crap on the market.
I’d love to be able to default the price at NBBO +/- X isk, rather than having Eve contrive some abstract and typically wrong value. I’d even prefer an empty box over their suggested price, as I would only have to click the text field once rather than twice to type in the new value.
Lots of people just sell to the current buy order… I’m not one of them. I always place my orders, and having something that is “more accurate” would save me needless clicking. Market PVP doesn’t have to be something that makes you want to slit your wrists.