I have been watching the forum and other advert places for the last couple of years and I have never seen any for sale that is £350!! I might have missed them if there was but can anyone honestly say or prove that they have seen secondhand hw100 on this forum in good condition for less then £500! The last couple I got had faults and marks for £450. If it is really good condition even with the age I value it at £500.
If he is not your mate then you could offer him a cheap amount and you might get a bargain, but as for value I am sticking with £500.