Just to second what Beansprout has said here about there being no problem with credit cards. I have only once bought second hand and the condition of the book was not good but was honestly described. If you buy second hand though, you have to pay £2.75 p&p to each seller, but if you buy them all direct from Amazon, you get p&p free once you have gone over a certain limit.
I have never had a problem with delivery either. They have always been exceptionally fast - once the item arrived the very next day! Don't be fooled by them asking if you want the items delivered separately as they become available (they will charge extra for this and even if you agree to wait until all items are available, they often send them separately, at their expense, anyway).
I dond't only use Amazon, I also use other online book shops, but Ido think Amazon is generally cheaper than others.