Kinda of a tricky situation you got there. On the one hand, you don't want to be an arse and totally ignore the lady or flat out say 'No" to her request even though it's well within your right to do so as she defintiely should have done her research better. But, at the same time it's not your fault that she sold something for less than it's worth so why shouldn't you just keep it as is?
Personally, if it were me, I'd go the already suggested route and offer her another $20 (or somewhere around there) if you can afford to do so. But I'd do so with the clear, but polite, understanding that it's that or the deal stands as is. She's having a bit of seller's remorse after finding out what things are really worth once it's too late (has happened to me before so I know the feeling). It'd be nice of you to make such an offer, but in no way is it required.