This is something I've always played with but had never felt like doing until now. Making premade Choppers to sell kind of follows along the same lines for me as stockpiling: Investing a relatively large chunk of gold into one thing and you sure as hell better hope it sells.
Now, however, with Bartering available I have decided to take the plunge. I'm using all of my stockpiled saronite to transmute Titanium Bars. I had anticipated Eternals going up in price so I stockpiled many Shadow and Earth and a few Fire. I quickly ran out of Fire, but have been able to buy cheap Frozen Orbs (~9g) which you can turn in to Frozo the Renowned in Dalaran for Eternal Fires.
With Bartering I pay 11,250g for the vendor parts of the Chopper. I always put a premade one up on the AH for around 17000g and offer in my bark to sell cheaper than "my competitors" on the AH. If people ask I sell them for 16k and have sold four this way, resulting in what I peg as about a 4.5k profit per bike. (1,000g is a bit high for Arctic Fur and Titansteel, but I like to be conservative with estimating my profits.)
When someone requests that I make it with their mats I offer to Have Group, Will Travel them to K3 (I can get there much more swiftly than they can thanks to my Wormhole Generator which puts me at Ulduar or my Kirin Tor Ring. I seem to get a lot of "Wow thanks that was fast!" responses, which may be why I've never been tipped under 1,000g per bike. I do not tell them how much I need for vendor mats (They usually have done their own research to come to 12.5k) but if they ask I tell them "It's 12.5k but I have Bartering" and link Bartering. Inevitably they've always given me 12.5k and allowed me to keep the excess in addition to a tip, which is some great extra gold for the craft.
What's to become of this?
As we've moved further and further from Wrath of the Lich King the materials for these have become scarcer on my server; particularly the Eternal Fires, as they're not often gathered by people leveling mining. They're up to roughly 60g a piece on my server and will probably continue to climb. Smart gold makers I think can capitalize on this by knowing roundabout ways to gather mats (transmuting Saronite into Titanium, trading Frozen Orbs for eternals, trading Heavy Borean Leather for Arctic Fur, etc.) and keep their production cost low, allowing them to always be able to beat their competition's price.