I don't know how reliable paypal is. I'd use it if the amount was less than £50, otherwise I think I'd ask for cash on collection or a bank transfer.
Last year, I sold some bits and pieces for a friend of mine. Several weeks later, after all the items had been posted, received and positive feedback given, I got an email from paypal telling me they had put a block on the funds for one of the items.
They asked for proof of posting and proof of delivery so I sent them both. Then they proceeded to ignore my messages to them in which I asked them to remove the block on the funds. After several days of no response from them, I got bored and issued county court proceedings against them and the buyer, using the money claim online service. I then sent another message to paypal, letting them know the county court case number.
Paypal entered a defence, in which they said they were unable to ascertain the actual paypal account or transaction involved, despite my having messaged the details to them at the outset. It was only after the matter had been transferred to the buyer's local county court and a hearing date allocated, that paypal removed the block on the funds, and paid the court fees into my paypal account.
They never did give me an explanation as to why they had put the initial block on the money.