Have you tried it with another camera?
I'm sure you have a friend or relative that can let you try it on their camera.

Also what about all her previous customers, from the look of her thread, she has quite a number of customers. Maybe you can PM them regarding this issue as well.

Anyhow, she has offered to refund you earlier as you have shown in her text message, so I don't think she should say that goods sold are not refundable after so many months. If her policy is that she doesn't accept refunds then she should not have offer it in the first place.

Seller should also find time to come out and meet with the buyer to see what's really wrong, why she test on other camera can and why yours cannot.