I buy/sell face to face on anything £50 plus for this very reason.
If I brought something and missed something as others have said my bad! I would take it on the chin, if I missed it the seller probably did too and so assume it was sold in good faith.
If I sold something and they missed something then generally that’s why they were allowed to inspect it, your ad looks genuine to me with more photos than most…you allowed a good inspection and he was happy with it, it’s his problem now he needs to accept it.
Personally I think if you got £1400 it was a damm good price for you for what looks like a very well used DH bike. But he saw the ad, saw the pics, inspected the bike and paid the cash. I think he’s a chancer or has buyers remorse due to the price he paid!!