Did they already confirm on poshmark that they received the item and gave you a seller rating? If so, then all sales are final and there is nothing the buyer can do. At least, that is from what I understand. I'm not sure if I have heard of anyone opening a claim after the 3 day deadline, unless I am wrong...?
Worst case scenario if she is still within the 3 days she opens a claim and then Poshmark asks her to send the item back while they refund her. I don't see how they would side with her, though, especially if you posted your own pics...
I'm sorry, I don't know if I can add to this and I know I didn't answer your question. I was actually in a position where a girl bought an item from me via Poshmark and gave me a high rating...but then the day after started questioning the authenticity of the item I sold her (!!!). I kept affirming that the item I sold is authentic (and I know it is because I was the first owner and it was straight from the boutique) and I kept stating she could always authenticate it with my permission. She relented eventually, but it was nerve wracking. I honestly don't even know if she even bothered to get the item authenticated.
I actually really
don't like Poshmark that much because I feel like seller protection is less than nothing (whereas with Tradesy, they offer some protection when a buyer isn't happy with the item). It also doesn't help that the maximum number of photos you can upload is 4. I like to take lots of pictures because I know people don't read and I like to show buyers all the flaws and details of the items I have posted. And I just think it gives more wiggle room for remorseful buyers to fib about what they received.
I tend to sell more stuff on there than Tradesy, though, which kinda sucks because I'd rather sell through there than Poshmark.