FWIW since 2001 I've sent almost 7000 packages through Royal Mail bog-standard services. I only had an issue on two occasions and both times ultimately it turned out to be a problem buyer.
Ask the buyer to return them so that you can test them yourself, given that you know they were working last time you checked them and to show you are acting in good faith. If they get returned to be test you know the buyer is acting in good faith even if it's just a case of not installing them correctly in their sockets. There is a slight possibility that a switcheroo has occurred but people like that tend to go for switches they can repeat over and over again, the chances of someone having a pair of identical damaged vintage tubes to switch for yours is very slim.
Speakers: Wilmslow Audio Prestige, ATC SCM11, B&W 602S02
Cables: Missing Link Cryo Ref, handmade pure silver Litz braid, Van Damme Blue
Amps: Hypex nCore400, DIY SET EL34/ECC83, DIY PP KT88 Monoblocks, Hypex UcD400 HG
DACs: Quad Deck DDDAC, Single Deck DDDAC, Modified Chinese 9018, DIY Russian TDA1543 DAC
Turntable: Technics SL1200 Modified, linear PSU, Paradise Phono Preamp
Interconnects: Handmade silver Litz, Missing Link Alaqeia
Preamps: DIY - buffers, LDR attenuators, stepped pots
Mains: Balancing transformers, RF filters, handmade cables