I voted, but will elaborate a bit. I have a hard time turning down work, and am not so consistently busy that I feel I have an option most of the time. So I do customs, build knives on "spec" to sell, and will occasionally send a few to a dealer.
A lot of the time what happens is I'll build a knife however I want to, post it for sale, and it's taken. Then I'll get some mails from folks who either want the same knife, or want the same knife with different handle, grind, etc. Or they may have liked the knife that was for sale on the forums, looked at my site, and picked out a completely different knife that they'd like a copy or "version" of.
Most of my custom orders are versions of knives that I've made. I don't usually make the same knife twice otherwise.
Another bracket of customer is the guy who sends me a drawing or tells me an idea, then we work the final design and materials out. Or they'll send me pics of a bunch of factory blades with different aspects they'd like to combine into a knife. I try to retain some artistic license with this type of work.
Sometimes I'll take factory blades and outfit/sheathe them. If the money is right. I don't usually mark a blade that I did not personally heat treat. (Don't want unknown factory steel/methods blighting my name if they fail.)
I just finished all my custom orders yesterday, and am going back to the "drawing board" phase.
I'll add: most of the knives that I make on "spec" are enjoyable and/or challenging to make. It's nice if you have a portfolio of finished knives for customers to look at, which have been designed and built by yourself. That way, it's a knife that's enjoyable to build twice or more, the design makes sense to you, and you have a good idea of where to price it. I also find that customers make their minds up more quickly and with less wacky ideas when they are looking at a bunch of knives you've built before and "picking" one. The most they may want is a change in handle material or an upgrade to mosaic pins, etc.