What I do is as soon as I buy a batch of kits, or blanks I enter them into the database at that time along with what each item cost me, and most of the time add a sample picture to the DB.
Then when I make a pen, I remove the kit(s) I use, and the blank(s) i use from inventory.
Most of the time I am making stock inventory for the shows or to add to my site (as soon as i get it done), and i usally sit down and plan out 12-24 pens of different types, and different blanks at a time, I set up the kits and the blanks together in a plastic bin cases I have.
Then at the same time I create a record/part number for each of the finished pens, this also helps me keep track of the actual cost of materials for each pen.
The DB also tells me when to reorder when stock gets low. if I set the DB record to do that (kits and blanks I set up that way finished pens i do not)
I try to buy stock for the stuff I sell in the shows in quantity so I might have gotten the kits before at say five bucks (for example), but the next time it is 6 bucks this way I can adjust my prices for what the materials replacement cost are at the time I use them, and sell them.
plus the DB generates sales orders, and purchase orders. Not to mention keeps an address of customers and vendors.
I know it sounds like a lot of work but it isn't really, once the DB is all set up its just takes about 10 min. or so each time i buy stock, or make pens. Plus I can generate reports to see if I'm making profit or loosing my butt.
