Your response seems perfectly fine. And until Posh gets back to you, there's nothing much that you can say to defend yourself. Just hang on tight (I know that's hard to do).
Even with Posh Concierge, it's not always guaranteed that you'll get what you've order or as the seller, it's not always guaranteed that you'll make that sale. I am not saying that you didn't accurately describe the item but from my personal experience, Posh only authenticates, they rarely make note of the condition of the item. Currently, I am in a middle of a return request and the item went through PMHQ. I ordered a brand new Chanel, it got approved by Posh and I just received it yesterday. Upon inspecting the item, I noticed that there were quite a few light stains and the item had traces of glitter. The hardware showed no signs of wear, so I knew the item was new, but it was not properly stored and that's how the other marks probably came to be. Now, I am sure my seller thinks I damaged the item to get the return, but that simply isn't the case. I am not even upset with my seller. I'm a seller and I know sometimes I overlook flaws, but that's a mistake anyone can make. I am extremely upset with Posh. The fact that they provide this service to only authenticate but not fully inspect the item. Not only does it wastes my time to have to file a return for this item, now I look like the bad guy to my seller.
So maybe the imperfections led the seller to believed that she got a used bag that passed Posh concierge service. That is really all that I can think of in terms of Posh not doing its fair share.
Buyers typically do not communicate with sellers when a return request is opened; it is unfortunate. The reason being is that sellers can't issue refunds, only Posh can. So to communicate with your seller is sometimes seen as useless.
On this note, yes they only authenticate the bag, but that's all the service is. They don't know wether or not you care about the glitter or the marks on the bag, they only look at the authentication. Bear in mind, they actually video each and every package they open so there's no question that the bag was tampered with or the bags were mistreated or switched for an unauthentic bag. Sucks that you received the bag like that, but make sure you don't accept the bag if you get it and it's not what you've expected!