Buying directly from Amazon is very safe -- I've bought dozens of things from them over the years and the only problem I've had is that they shipped 2 of something once (they took the return with no questions).
If you buy from one of their used book vendors (the ones listed under "Used and new copies available from ..."), choose one with the highest possible feedback -- 98% positive or above.
They do not accept PayPal. "Amazon.com accepts American Express, Diners Club, Discover, JCB, MasterCard, Eurocard, Visa, Visa Check Cards, payment from your bank account, Amazon.com gift cards, Amazon.com Gift Card claim codes, Amazon Store Card, and Webcertificates." This is from the Help section, which is linked at the upper-right corner of every Amazon page.
Hope this helps!