I just received an email from Greentoe after making an offer.
This isn't readily disclosed in their promotional information. It seems a bit sketchy to me.
"If you increase your offer, the visibility of your offer will increase as well. For example, if someone offers $100 for a product that is $1000, their offer will not be seen by our retailer network. Someone that offers $700 will have generate more visibility than an offer at $500. To sum up, the higher your offer, the more retailers will see it."