If you don't have something listed, you can't cater to impulse buyers. You want to cater to them, because if they can't buy it now, they will find somewhere else to buy it. In the time it takes you to respond to a custom order listing, they could have already found it elsewhere.
I experimented with offering some made to order items in my shop. Overall, I do think it is working, people understand when they buy the listing it takes 1-2 weeks for me to make. I have only done it with my popular sellers, and I plan to add in more, as I really think it has helped with impulse shoppers. I am going slow in adding them, as I want to make sure I can keep up with demand, so far so good, I probably list about 1 style a week as a set.
Here is an example of what I am talking about.
http://www.etsy.com/listing/93237085/sock-monkey-children-decor-lighthttp://www.etsy.com/listing/93237032/sock-monkies-children-decor-light-switchhttp://www.etsy.com/listing/93236947/sock-monkey-nursery-light-switch-plateA customer can just pick out which one they want to buy, with out having to convo me and tell me exactly what they want and me setting up another listing for them.