Without going into the (otherwise interesting) definitions of 'new items' and 'open box' it doesn't matter who the retailer is....if the warranty and return conditions are substantially different with an open box item you have to ask yourself if that makes up for the small difference in price.
It is outright impossible to predict if any given motherboard will perform as expected. As in your specific configuration and set-up, let alone boot up and all the parts functioning correctly on every mobo made...new, used, open-box, refurbished, second-hand or lovingly used daily like a bangkok working girl...either the boards work when you turn them on or they don't.
Since there are no guarantees, nor predictions possible make sure you have the best return/replace/refund policy for your needs..maybe the open-box suits those needs and you can save a few bucks - it's a good mobo model.