I want to change via web service a remote inventory, I know that via Event Observer Method can triger my code, but I don\'t know which event is useful to complete my task, l
I did a blog post about this (which contains the full event list for Magento CE 1.4) a few weeks ago.
The events that may interest you for an order placement is sales_order_place_after, which gets called after the order is placed (seriously!).
sales_order_place_after
Hope that helps!
Thanks, Joe