There are numerous ways to make gold with engineering, such as creating pets, ranged weapons, scopes or even gnomish army knives.Do you want more gold tips? Don't forget to share!
The best one of them in my opinion is the highest level gun they can make, the Kickback 5000 which is learnable from trainers at skill level 525. I'm usually the only one selling it because most engineers seem to prefer it's almost equivalent, the Overpowered Chicken Splitter over it, which in fact is less superior due to the missing red socket and socket bonus. This means I can choose to sell it even for 20k if I like to try my luck, though I'm happy when it sells for 10k or even 5k.
Kickback 5000 - Material costs
3 x Truegold
9 x Hardened Elementium Bar
1 x Chaos Orb
And some junk you can buy from the vendors.
I've made at least 200k total from selling it alone in the past months. Everytime I advertise it in the trade channel at least one potential buyer whispers me. Then if you are silver-tongued, the deal is already finished, assuming they have the gold. All you have to do is convince them why it's the best weapon they can get without effort until raiding, and why it's worth the gold.
These days the mats costs rougly 2000g to make + one chaos orb. I usually sell it for 5000g-9999g depending on competition or if I sell it in /2 instead of AH, so I get atleast a 3k profit everytime. Most of the time even 8k! It sells fast considering the high price since it's the best ranged weapon available for newly dinged hunters. It's ilvl also makes it a great pre-raiding weapon in general.
If you want to sell it faster, it's a good idea to insert a Delicate Inferno Ruby into it's socket so it looks even better with the extra 40 agility and 10 from socket bonus. If you are selling it in the AH, don't bother with a scope, as enchants will not show up on items in the AH for some reason.
You should also advertise your guns in the trade chat to sell it even faster and to avoid crazy AH cuts and deposit fees. Just don't forget to make your message catchy! In this case you should create a Gnomish X-Ray Scope for it to make it even more good looking.
Remember that "WTS <item here>" alone won't catch as many eyes as "Are you a newly dinged hunter? Want to replace that crappy bow? Fully enchanted and socketed <Kickback 5000 link here> will last until you finally get the bow from Atramedes! /w me and start raiding today!"