I do custom wands, and I often find customers' imaginations are much more vivid than reality allows for. I just make sure to be honest about my abilities, workload, turnaround times, etc.
I've generally had good experiences. I always think if I was ordering a custom wand, I'd have a design in mind, but most people are more concerned about the species (it's a Harry Potter thing), and need some guidance through the design process. I keep drawings of wand ideas and past designs, and most customers end up going with a combination of things I've done previously.
The trap I try to avoid is buying materials or equipment that I don't already have on hand and wasn't already going to buy. Unless I have the stock on hand, I (now) let the customer know that they'll be paying for all the materials I buy, even if they don't all make it into the project, because I can't always control the dimensions of the stock available to me.
Bad example--a customer requested a wand made of sycamore. It's a lovely wood, but not one a lot of people are looking for, and given how light the color is, I wouldn't anticipate it being a great seller for a pen. I took the job, and had to make a 45-minute drive to the nearest hardwood dealer, where I had to buy a 3-foot by 8-inch length of 3/4 stock. I normally like my wand stock to be at least 4/4 thick to give me more options for turning to shape, but all they had was 3/4, so it was that or an even more expensive internet order.
Long story short, I was left with a bunch of wood in a thickness I don't like for wands and a species/color that won't sell if made into a pens.
Had I been dying to work with sycamore, the expense would have been worth it just for the experience, but as it is, I think I barely covered costs on that particular job. Not a problem when it's just one project paying for the next, but not a great business model, and it certainly doesn't help other makers who are trying to competitively price their own work.