When do vendors (e.g. Delanynder, Expat, Opold, etc.) get new items? Is it after a specific number of missions or assignments are completed? Or a specific amount of in-game time? Or...?

Most of the Merchants in the Citadel will replenish their supplies after you leave and then return to the Citadel - some will have new items based on leveling up - especially Medi-gel and Grenades upgrades.

The Requisition Officer on the Normandy will replenish his supplies when docking on a plant or space port (seeing that the only planets/ports that the Normandy can dock on are: Noveria, Virmire, Feros and The Citadel Station - it is either one of these, or on some additional planets - I have yet to test when, where and on what occasion this happens). Silverstrike 12:40, 22 December 2008 (UTC)