Bloody hell! If she does that she truly is a nasty piece of work! She sold it on Gumtree so I assume there is a record of the transaction (?) So would have a hard job proving it was stolen from her? I was surprised she sold it. Maybe she bought it just for the meet.
Unless I was duped (again) I spoke with the new owner and I believe their story. They were annoyed that the phone could be potentially stolen. Didn't occur to me that she could also do that.
It's strange since eBay owns Gumtree, but Gumtree has a notoriously bad reputation for scams and dodgy goods. eBay offers much better protection for customers but Gumtree is like the wild west.
What kind of phone was it? Samsung Galaxy? iPhone?
If it's an expensive phone, I can understand why the seller is worried.