I would trust your instincts regarding the unusually high bids from Chinese buyers who just happen to have "agents" in the UK. If it doesn't feel right then stop the sale.
Before sending any valve gear via the Global Shipping Program, you should read this post on Tapeheads, which was posted a couple of years ago:
eBay Global Shipping Program Seizing Tubes! (sorry I can't post a link here but a search will find the post)
I don't like the Global Shipping Program. It's very expensive and, on the one occasion I used it when buying a rare chip from the USA (the seller would only send GSP), the delivery in the UK was handled (badly) by Yodel. Generally if a seller will only use GSP, I walk away.
One thing I have to watch out for, as a seller, is Ebay setting GSP as the shipping method on my account without notifying me. I bet this happens to a lot of sellers.