Why not be honest? Just tell her that her buyer profile doesn't meet your preferred criteria for buyers of your specific items, and leave it at that. It is your absolute right to deny certain types of buyer from purchasing your items, and eBay can only agree. Also, add her user ID to your BBL so that she can't buy any BIN items you may put up for sale. I blocked a buyer just the other day because although his two offers were rejected, when I checked his feedback I noticed that of the feedback he'd left for sellers, all 5 of them were negatives, and I wasn't prepared to take the chance that I may become the sixth.