Just ask to scan.
If you don’t know what to say, then just ask to scan. On a regular basis, I have people that I will approach three times, and they’ll say no each time and then I just ask if I can scan the shelf and they say, "sure."
Try that this week!
On a slightly more technical note, I ask to scan by saying, “I’ll just scan the shelf and pull them out a little bit if I’m interested. You may push them all back and I will not be offended. Is it okay if I at least scan the shelf to see if there's anything I can send to a student?”
If you think about it, the people that are harder to approach probably have more books, because if they're shutting you down, they shut down other buyers as well. Push through, and just ask them if you can scan. "Honestly, it's totally fine if I don't find anything, but I told myself I'd scan 15 offices today. Do you mind if I just check to see if I can find anything? I'll slide them out a bit if I can use them and, at the end, you can literally just take 30 seconds and push back in anything you want to keep! Do you mind if I do that quickly?"