Products and services do not disappear from the marketplace without any reasons. The only explanation why items can appear to be missing is, if they are marked as inactive by the seller or if an item is sold. Then those items are listed in the respective categories: either in “inactive articles” or in “inactive articles” and “sales” at the same time.
Please be aware that an item listed with a quantity of only 1 unit will automatically “disappear” from the marketplace when it is sold, because it moves into the “sales” and “inactive articles” category. To prevent this and in case you want to offer many units of the same item, please ensure to increase the quantity of the product or service within the item description.