I have been on ebay since September 98, and have bought close to 400 items ranging from cars to manuals.
In addition to what has already been said.
Some things to watch out for: If an item only accepts pre-approved bidders do not email asking for permission to bid, a lot of these are just trying to harvest email addresses.
If you are asked to enter your password check to make sure the website starts with https://signin.ebay.com/
Do not depend just on someones feedback rating, look at the items they have bought some of them will have 20-30 $1.00 items all in a short space of time just to build up feedback. If the items are not viewable check the sellers and see what kind of items they are selling. Do not believe feedback from people selling recipes that they email to you etc.
On a big ticket item ask for the sellers phone number, get their name, call them, check an online phone book make sure Name, Phone and Address match. Stay away from college dorms, apartment buildings etc. if the item is not something that an apartment dweller normally owns.
If the item is distant, post on this board or others you frequent to see if their is somebody local that can go look at the item for you and report back (offer to pay mileage and meal if appropriate).
'05 JD GX335, '96 Sabre 1538HS