Because sellers are able to see what people are wanting to pay for their item (its included in the listing), they can easily identify the going-rate for items and use that to decide whether they want to sell or not.

Like any marketplace there'll be time wasters - I don't sell on eBay anymore because of them - but in my experience, most of the time its the buyers that are time wasters.

In our case - since buyers have gone to the effort of creating a WTB listing, you could argue they're less inclined to waste sellers' time. Of course, the feedback system/reporting users would help us to identify repeat offenders and take action.

Check out our demo which should help explain things: http://www.wantobuythat.com/demo.html