Think how much you would be happy get if the auction ran. If the auction ended at £5 would you be happy or pissed off?? Keep going up until you reach a figure that you would be happy with then charge that.
You don't have a buy it now if you don't want to just reply saying you want to give everyone a fair cahnce and let the auction run it course. Alternatively ask him to make you a reasonable offer. Then you will get an idea how much people are prepared to pay.
If you decide to set up a Buy it Now then you will have to do ASAP as there is a point when you can't do it anymore. Try in revise your item and see if the Buy it Now option is available.
HTH
FB x