I just thought of a random idea so sorry if its not worth the time etc. Basicly, if an item is out of stock perhaps you could have amount expected to arrive so say if you ordered 100 units of item A then youd expect that 100 to be in at the ETA date given, you could also add a system for saying how many people have ordered it while its out of stock so for example, Item A is expected to arrive on the first of july and you have ordered 100 in and currently 80 people have ordered one while its out of stock, Person A sees that their could be 20 avalible if he orders now so he orders it, after this person B looks at the item and see's that there is only a few left and decides not to take the chance as he needs it soonish and so looks for an alternative.
Not sure how much effort it would take to implement though so i guess the idea wont be really useful, but its something you can add when you get time .