Actually, I've not sold a pen yet that wasn't pre-ordered. I have all the pen blanks in a box and I show them my pen and a few pics of the types I make. They choose their wood and pen style. I write an order blank for them and write their name on the blank. Then I make the pen from that piece. If I run out of that type of wood in my little box, I replenish it from my wood at home.
Maybe soon I'll get quick enough at these and have the float from previous sales to make a bunch of them in advance and sell. But for now, since it is mostly co-workers, friends, and their friends that order them, it works out.
I guess it lends itself to the whole "handcrafted" approach. They new they picked that pen blank and I made their pen. ;-) whatever works.
Mind you, I'm selling them because (a) I love to make them, and (b) they earn enough to pay for the hobby as well as help me pay Mr IRS for a couple mistakes on my 2002 taxes :-(.