The Mote extractor remains the Engineer's unique resource extraction tool, and so remains a source of revenue for the engineer in the Wrath of the Lich King expansion. End-game level guns also remain a source of wealth.
Surprisingly, pets make a come-back as a source of income due to the pet collection achievements. While there was an early market for the motorcycle mounts (among the most wealthy on each server), this market dries up quickly. The remaining people seeking them will generally have a guild member who can craft it for them (given sufficient resources) by the time they can afford it. On the other hand, with level progression available to 80, fewer engineers are gaining the Consortium reputation required for Elemental Seaforium Charges, expanding the market for those who have the recipe. Typically, though, this will be a 'tips' market, as the engineer seeking parts for her flying machine will be able to provide all the parts for the charges.
A slight resurgence of quest-required items can be expected as people go back through low level quests while working on their Loremaster achievement.
Note that there are clouds of Motes in several regions of Outland as well. Market for those, though, is largely for players practicing their trade skills; end-game materials has moved on to the crystallized materials.
While there is some market for scopes, the speed of leveling means that scopes are often overlooked until quite late. And once a hunter has his ultimate weapon, he will typically only need a single scope.
With achievements for the number of companions one has collected, engineered pets are much more in demand. The Lifelike Mechanical Toad is a particularly rare recipe, and the pets created with it can bring a premium from hardcore pet collectors.
Services are using an item to do a service in exchange for money
Lock Opening
Using Seaforium Charges you can open locked boxes. Just like a rogue, usually you do it for a fee or tip.
Mailbox or vendor
Occasionally a player may need a mailbox or vendor in an out-of-the-way place and will pay a small fee for you to use [MOLL-E] or [Jeeves]. Places where it may be required as there are no such facilities nearby: in a raid or dungeon, Darkmoon Faire at the opposite faction's zone, items for large turn-ins of rep such as Ravenholdt which may be stored in the mailbox.
Everyman Items
These items do not require a skill to use or equip. The majority of them are in low demand, though.
These items are crafted by engineers, for engineers. In most cases, they are specialization recipes (gnome or goblin) that do not require that specialization to use. In other cases, the recipes are reputation awards, and the products can be sold to those who have not yet done the required reputation grind.