Otis has been using the archive function to relist the same item, so that it appears on the TOP OF THE FEED (rather than promoting). He can't seem to keep track of which items are sold and "not sold" within his archived folder because he has 100's of items, hence he is trying to explain the desire to have a DELETE option.
OfferUp wants people to spend $$$ on the promotion feature, rather than the method you are using to repost items. It would work against OfferUp to implement a "delete" button.
FYI, people have been asking for the delete option since its inception...don't hold your breath.