I never bring my good pens to work. I buy the cheaper ones and keep tons in my locker and 5-6 on me. That if a customer asks to borrow a pen I can let them use it..
Although I only lost about 2-3 pens via customers borrowing them. Rarely will a customer keep my pens.
I loose more when I let co-workers borrow them or I leave them laying around after using them.
Read "customer" appearance carefully. Note douchebag or lady-acting-all-sweet appearance. Maybe say no. Let the manager lend out pens. Also, if pen is loaned, you have one job - to get it back. Eyes on pen. Provide assistance with pen return as needed.