One downfall I see is if there are people who sell things regularly that don't end the price in 0.
Ex. Someone sells something for 50,341, not even knowing people do this HQ pricing thing.
Also, what's to stop people from raising a price on an item and adding a 1, 2, or 3 on the end to make it look HQ when it's really not?

I suppose there are risks you take. :\