Marineparts - I have used them and have no complaints.
Being stuck in a basement in London I can only get to the chandelries at the weekend. Normally when I want to be on the boat and not browsing for lifejackets. I've used the web almost exclusivly to buy bits for the boat, and to buy the rest of the stuff you need at home etc... Last Christmas was great - I didn't go near a shop (other than an off-licence) in December.
Couple of things were noting: Look for a padlock in the bottom right of the browser to ensure that the connection is secure (normally 128bit). Always buy with the same credit card as they offer their own protection / insurnace - and if it does start to get abused it's pretty easy to see. Chase the goods if not shipped to you in 3-5 days.
Worst web site for boats - actually YBW bookshop - but improving I hear.
Worst general site - John Lewis - nothings ever in stock and they lie when they say they have shipped.
"No, No, no - come off the plane before entering the lock..."
I buy Cd's on the net, books, air tickets, hire cars etc. I bought my last boat on the net and a year later a 100HP Suzi for it - again on the net.
Of course, I am biased because I work in the Inetrnet business but seriously though, when buying, the vendor should alert you to the fact that the site you are buying from is secure.
You will know if the site and the information you are sending to it (ie Credit Card numbers) is secure because as you perform the transaction, a small gold coloured padlock will appear in the bottom R/H side of your screen, near where it says "Internet Zone". If you hovver you cursor over this padlock it will give the encryption detail, it may say something like "128 bit encryption" or something like that.