'And in each case I did take a picture of the buyer holding the item, so I do have proof (in addition to the message trail) they'd come to collect it. '

Sorry but you have no PROOF at all. eBay do not have photo ID of anyone using the platform (and even if they did, they would not be able to use it). How do eBay know that your photo is of the buyer? It could be your cousin `Bubba` for all they know!