The rules seem to have changed so that, from 29 January this year, it's not been possible to specify 'buyer collects' (or similar):
http://pages.ebay.co.uk/help/sell/maximumPP_FA Q.html
Reading the information, on that page, under 'Are there any exemptions?' suggests that the best you can do (to indicate 'buyer collects') is to select �Courier : Heavy and bulky items� from the drop-down menu and then use the item description to make it clear that you'll only sell to a buyer who arranges collection.
Chris