Hi! I'm Grimlynn, gnome warlock on Maelstrom (RPPVP). I got a quick start on Mining and Engineering when I made my character, and I've actually made some decent money for an early character with it. I've found that the people on my server love the following:
Rough Boomstick and Crude Scope: These two items will be the first things you can make that are worth selling to other players. The Boomsticks cost 3s plus materials to make, and I sell them for 6s each to newbies (with a Crude Scope attached) and to Enchanters, who buy them in batches to DE for materials. 6s may not seem like much, but when an Enchanter is buying them ten at a time and you're untwinked like me, it adds up fast.
Mechanical Squirrel: Everyone loves these cute little things, but you have to buy or find the recipe; I won it in the AH. The two Malachite are the only troublesome part of making them. 40s each (same as other little pets) seems to be the going price, though I've heard of prices up to 1g each on some servers.
EZ-Thro Dynamite: Here comes the BOOM! High explosive AOE fun for everyone, but you have to buy or find the recipe; I won it in the AH. This is only a moderate seller; EZ-Thro II is similar, but for higher levels.
Deadly Blunderbuss: If you play on the Horde side, as I once did, you can make good money selling these, because they are used for a Horde quest in Ashenvale.
Other guns and scopes: As you level up, you will find your crafted guns losing profits to dropped or quested guns, so they lose some value. But scopes never go out of style, and you can continually make money off them as you level up.
Quest items: There are several quests that involve Engineering items. You can make money in the AH from the specific items associated with those quests.
Materials: Blasting powder sells to twinks, and stone and metals can fetch good prices from anyone. So when all else fails, go mine a ton and load up the AH.
Hope all that helps!