Early issues (#1-10) more difficult to find. Annuals (1972-1979) and Summer Specials (1972-1982) very plentiful on eBay UK apart from the first one maybe and the last couple. The 1977 softcover Book of Gags is harder to find.
Apr 2014: a copy with the free gift Gulp! drink sachet free gift fetched a world record £659 on eBay UK. Well done Phil Shrimpton of philcomics! See the issue entry for a picture.
Note: some PR issues may have no value. Selected issues in FN, VF or VF/NM may not exist in some titles.