When I'm starting a build, I often first gather all the needed parts before building (due to the way I store my LEGO,I don't want to stand up for each part )
I think it would be handy have a page with the partlist with part, color and needed quantity, and behind that a column whit a checkbox that you can mark when you have found that part, maybe with a textbox where you can fill in the number of parts you have found if you don't have them all yet. Ideal would be some kind of refresh-function that refreshes the part-list and leaves out all parts that you have already found. For commercial purpuses: add the link that makes a BrickLink/BrickOwl wanted list of all remaining parts?
Hope I'm explaining it clearly enough like this