I am contemplating buying a secondhand Cartier watch, and the owner has offered to go to Cartier (on 5th Avenue in NYC) with me to have the watch authenticated in front of me before I buy it. She said they will do this, and that it might cost $100 for the authentication. (She said when she called the general Cartier number they did not mention a fee, but that the store on 5th Ave in NYC said there's a fee. She said they do it all the time for insurance purposes.)
Has anyone ever done this? Can one really just walk into Cartier and ask them to authenticate a watch? I don't have experience with high end watches really, only handbags, but I am imagining walking into Hermes and asking them to authenticate a Birkin and having them shoo me out of the store.
I have no reason to doubt this person, by the way... she's been very forthcoming, etc. But I also don't know her personally and therefore would not even be considering buying the watch from her if not for this offer of having it authenticated in front of me.
Has anyone ever done this? Can one really just walk into Cartier and ask them to authenticate a watch? I don't have experience with high end watches really, only handbags, but I am imagining walking into Hermes and asking them to authenticate a Birkin and having them shoo me out of the store.

I have no reason to doubt this person, by the way... she's been very forthcoming, etc. But I also don't know her personally and therefore would not even be considering buying the watch from her if not for this offer of having it authenticated in front of me.