I hope I'm not being too abrupt by jumping in here. Just thought I'd like to share my experiences with vBulletin skins.
First off, thanks to Chris M. for listing me first.
Anyways, in the long run, from my experience, providing per-client custom solutions has generated far more income than pre-made skin sales. With that said, the work-to-money-earned ratio for pre-made skin sales is very nice and is a great source of (relatively) passive income.
I've tested the waters with selling made-for-mass-consumption styles a few times with a few skins over the past two years. It seems to me that while it may be lots of fun waking up to a hundred PayPal payments in your bank, it becomes extremely hectic once Jelsoft updates vBulletin and you try to update a dozen styles within a couple of hours.
In addition, providing support for non-custom skins is a major pain. There is always the 10-year-old kid who somehow came up with the money for a vBulletin license and demands support for every little template tweak and nags on you 24/7 to change an element of the skin or add a feature that he wants.
Designing custom skins, on the other hand, is not only great fun, but also becomes more profitable than reselling skins, in the long run. Contrary to smacklan's post, I will heartily disagree about the business potential of doing custom work. Providing custom forum skins is not too different from designing a custom website. I've worked with many clients who needed not only a custom vBulletin skin, but also server setups, backend-programming, and even database administration. Contrary to what's been mentioned by just about everyone in this thread, there are companies that are more than willing to spend tens of thousands of dollars on their websites. In my experience, working with companies on a one-to-one basis provides more income with a faster turnaround time than simply selling pre-made skins to mostly individuals and small companies.
So like Smacklan, you agree the money to be made is in custom work, rather than mass-made-skins which are sold over and over to everybody. But on saying that, do you still make a nice earner out of mass made skins though?