In respect to the job position I do up sell. I always see if there is something else that can accompany a purchase such as if you are buying placemats don't you need the matching napkins or a nice centerpiece or candles to you table?
I know something of hardware (this was a hardware/home improvement/gardening store) and what to do with it from my years in technical theater, and while I wasn't an expert at any aspect of supplies or construction, I thought I knew enough to be able to refer customers asking questions I couldn't answer to whichever more knowledgeable salesperson did. I have also held a retail job as a cashier/clerk, and quickly learned how to fix our temperamental cash register. (I never had a chance to finish giving this answer.) Suggestion: You've probably addressed some version of this question while writing your cover letter. If possible, go over your cover letter/resume/application/whatever else applies right before your interview. It's like reviewing the material you're about to be tested on right before the exam, and will help you keep what you say during your interview consistent with what you've already told them during the application process. The last thing you want to do is contradict yourself.