I completely agree with all PPs suggesting that she wanted to sell it, but I can't see how this would prevent her from still doing so.
Unless she has a specialist pushchair shop and was hoping to pass it off as brand new (and even then, she could describe it as slightly 'shop-soiled'), if she could otherwise get £200 for it in perfect condition but knocked a fiver or tenner off because of the cracked clip, she's still many quids in as a result of a kind stranger's generosity. She could probably even just unscrew the clip and nobody would ever know.
Some people are just utterly shameless and see other people only as accessories to their own selfish desires - and are then actually angry at those people for their perceived rudeness for only actually effectively giving them £190 for nothing when they would have preferred £200. Additionally, some people will push and push for their own satisfaction as they have no concept of kindness and so assume that people who do show some humanity as stupid mugs who are ripe for the plucking.
I'll bet, if she won £5million on the lottery, she'd still complain bitterly that she wasn't offered her original £2 stake back - and would probably also demand her fee back for parking outside the newsagent to buy the ticket.
If it were me, I'd reply and say it obviously wasn't that big a deal if she only noticed it when she got home - and then add that, although you know that she's a decent person who wouldn't DREAM of trying to sell something that she asked for and got free from a kind stranger who was seeking a good home for it, if she WERE that kind of person, she would still be able to get a very good price for it. Probably water off a duck's back to her, but I still would.
Alternatively, as PPs have said, tell her that, as it's clearly utterly useless to her now, to sell it on eBay, keep her petrol money and the same again for her trouble and then send you the remainder of the cash after the fees. You will not hear from her ever again.