As someone who has also done a good stint in Oxfam shops, I do know what you mean about expensive stuff "disappearing" into a void when sent elsewhere.
I also ended up taking the dear things to sell elsewhere - at the time our shop had a Â£5 upper limit on anything.
The area manager was supposed to deal with good stuff but she was lazy and more than happy for me to sell stuff to local dealers I knew who were decent folk and gave good prices.
I researched the things for independant valuations, I kept records of everything, valuations and money paid, signed it all out of the shop, with counter signatures from another volunteer, etc. etc.
The rest of the staff were quite happy with this, mostly because the actual cash was credited to OUR shop, rather than elsewere.
Then the area manager left, a new one appointed. The shop was also appointed a new manager and the woman who did the bric-a-brac before me, a crony of the new manager, came back.
She didn't like working with me, thought she should be in charge of it all again.
(She was a dealer herself, in a local flea market.)
She accused me of cheating the shop and giving stuff cheaply to other dealers.
It all turned very nasty indeed.
I left, and went to work in another Oxfam shop elsewhere.
As for mentioning the GMB, it's only right to reference your source of information properly.
It also means if your buyer has any quibbbles about the information, they can come here and discuss it.
We're in the "business" of getting glass properly identified and it's makers credited for their work.
Apologies if we upset you, but there are a lot of sharks out there and we don't know you!
I think you should be very careful of doing good in an unofficial way like this, but I do understand why you're doing it.
What charity is it?