Then you should have her check out Lollipuff (YC W13): designer auctions pre-screened by experts for authenticity (ie. eBay without the fakes). It also amazes us. Why would people pay full price when the "lightly used" ones sell for 80% off?
FWIW, my wife was delighted when I sent her to Lollipuff yesterday. She thinks it's a great idea.
I guess one thought is that I don't know if I would ever gift her a used bag. Even if I trusted the source (which has always been a deal breaker before).
But her buying for herself, I can see her definitely purchasing something previously owned.
*Disclaimer: Am a founder of Lollipuff.