Caveat emptor may contradict Paypal's refund policy, but eBay would argue they are two separate companies...... caveat emptor is a legal pleading, and would probably apply if one took action in the UK, but eBay would almost certainly rely on the defence that they are only mediators in a sale, and not an auction house per se. As Paypal is a payment mediator, this legal device is irrelevant.
For private sellers, one has little recourse, and for professional sellers using eBay, they would not be any less subject to the trades descriptions and sale of goods acts within the UK, than UK charity shops selling items. I can't see any eBay professional successfully pleading that either act does not apply to on-line sales.
One situation where buyers should buy from eBay professional sellers, and take great care when buying from private sellers.
jmho,
Marcus