Depends what it is that is bought. If you for instance buy a layout and you made it but the 'buyer decides' against you already did the work. I don't think that's ok. And it's not because they don't like, but just because they are scammers and cheap people who want all for free. So ofcourse you should be able to give negative itrader for that.
Is that they just post your work and "neglect" to pay.. thats why I get a deposit. and thats what I see happening more and more. Joining a writing group has worked well for me for in print writing, but unfortunately there aren't a lot of union type things for online writers in that it hasn't gleaned the respect in many cases that it should command. That and we tend toward in fighting about our pricing and other things, as opposed to sticking together and setting our differences aside I think.. LOL now just WATCH someone argue with me and prove my point.