I would probably ignore the buyers question.

 am3

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As mainly a buyer, I think I would do what am3 says, if I were you. Unless you are willing to give a half way honest yet tactful answer.

But.. there is no way you can reply that won't give offence, short of lying and saying it was an error. IMO

I know I would be offended if I bid and then was blocked without explanation. So whichever way you swing it, she will be offended.

If I had a history of not paying, then it would be a good idea that a seller got honest with me, good lesson in consequences for me. Serve me right!

I am not sure many buyers come on these boards, so a lot would have no idea their FB alone could get them onto banned lists, they would probably assume if they were prompt payers they would never have trouble. Personally, I wouldn't classify 'Great seller' as poor feedback. Not everyone feels like writing a lot. I'd be happy with it if I sold something, put it that way. personally, I would not have blocked her but that is your call.

I know sellers have the right to block bidders unless they are happy with the bid.

Personally (although I think it will cause some offence) I would be inclined to tell the bidder why. To just say that your aim was always to have happy customers and your items were always honestly described,  but her previous feedback record indicated she might be hard to please and you simply wanted to avoid potential hassles.

She won't like that but it is the truth and will give her the heads up on the fact that her feedback might have consequences she had not considered. You might just make her pause before she gives neg or neutral to the next seller and not divvy them out lightly.

Just saying.