Exactly this.
PF48 is limited to £100 compensation, if you want anything posted then it is at your own risk, why should the seller have to stump up if it goes missing?
There is also the question of PCP's since they are considered pressure vessels and so prohibited as they are dangerous goods.
As a seller, I will offer anything for sale however I will state quite clearly that as far as insurance goes there is a limit and if a PCP is found and destroyed they are on their own.
I am willing to do an RFD transfer but at the buyers expense, again, clearly stated at the time.
It's up to the buyer if they want to take that risk but FWIW if it was my money I wouldn't.