The thing about boat jumbles is is that stuff sort of looks cheap 'cos it's on a wooden table in a field, but without a chandlery catalogue or the internet to hand, you can easily pay over the odds. I've bought heavy items like chain at a good price (would be costly mail order) but apart from that I'm a bit sceptical.
BTW for rope, email this guy (He's called Clive Douglas) and ask him for some samples. He's got loads more than he advertises, and if lots of people want the same type of rope, he will get a reel at a good price from the makers.
http://cgi6.ebay.co.uk/ws/eBayISAPI....ort=3&since=-1 or
clive@douglas01.freeserve.co.uk