I have bought things off ebay several times and I have yet to have a problem. Although, I would suggest you look at the sellers feedback number. If they have a high number of sales and a feedback of 98% or higher of positive, then I would definitely have faith that person is going to sell you exactly what they tell you they will.
If you are buying dvds, be careful they are not copies.Some vendors will sell you dvds they have copied themselves and the quality is not very good. I have bought copied dvds and while they were cheap and still watchable, you could definitely tell the difference.
If you are buying an autograph, make sure it is an original and not a copy or auto pen.
Shipping costs should be taken into consideration as well. If you are buying from another country, like for instance if you live in the U.S. and you want to buy from England, then your shipping costs will be very high. Be advised that most things are quoted in U.S. dollars.
You should set up a paypal account for your transactions. It is free and will guarantee payment. I do not advise you just sending them money through the mail as this is not a very smart idea. Always go through paypal as it checks out people and makse sure they are financially reputable to some degree.There are ways you can recover your money should you not get the agreed upon item and these steps should be outlined on the ebay site. However, I think the instances of this happening are extremely low. If you are dissatisfied with your purchase, you should contact the seller and ask them to refund your money and you will return the item and threaten to leave negative feedback and report them to ebay. This is not something I feel you will have to deal with. Ebay would not exist if the people trading on it were disreputable. The vast majority of people are trustworthy and honest. Just do your homework and check the sellers statistics out before you buy. People cannot manipulate their own feedback to positively reflect them, it is impossible. So, all of the feedback is genuine and trustworthy.
As I said, check the sellers feedback score and read what comments others have left and you will get an idea of how satisfied people have been with the purchases from this vendor. Make sure you compare prices between different vendors as well.
You can always ask the vendor a question regarding refunds, returns, shipping and condition of the product you wish to buy, the good vendors welcome questions.
Hope this helps you, good luck!