When buying anything expensive, I'd much rather do cash on collection. It's an opportunity to checkout the seller and the goods.
Also, there's no harm in asking a seller if they have proof or purchase.
I did this when I bought Carerra Intercity Bike - admittedly not off eBay (faceache market place), because at £50 it sounded too good to be true. Given the dodgy area I picked the bike up from, the suspiciously low price, I honestly thought I was going to have to walk away. The seller showed me the electronic receipt from
Halfords on his phone, which also showed the accessories he had purchased with the bike including the USB reachargeable lights it came with.