The experiences I have had as an ebayer may help some avoid the pitfalls. If you are interested, read on.
When buying or selling, always check out a persons record - 98% or more is great, but if they have only sold twice before, they are still a risk.
Read the feedback and calculate whether you can afford to lose the money you are spending/hoping to earn. Ebay are great, Paypal too, but it does happen. I sold an outfit which the buyer claimed not to have received. I had to pay them back and claim through the post office - another point, make sure you have a receipt for every item you send. I never knew whether they were telling the truth though.
Be as honest as you can. Give as much information as you can and ask questions in good time to get the answer and respond. The person you are dealing with may not be on the computer every day - I'm not.
If you do not know something, be clear that you don't so you don't get caught out or you will have to refund the money and/or return the item. Your rating will also go down.
Be aware not everyone is as nice as you would like them to be - but don't be that person!
Contact Ebay or Paypal as soon as you think there may be a problem. Don't wait longer than reasonable for an explanation from the other person.
Don't overload the costs of posting - you will be penalised. Make it as close to the costs as possible, including the packaging and a little for your expenses - petrol to post office maybe. Keep prices real. Don't ask too much - you won't get anyone interested, but be aware that you might get things sold on the first bid, so be happy about the price you start at.
Make sure your descriptions mention any faults, describe as clearly as possible and take good, clear pictures. My husband sells as a hobby and his success rests on all of the above! However, he is delighted if he makes 5p over what he paid for something. Its his gamble and he enjoys the little risks he takes. Every now and then someone really loves what he is selling and he may make a few pounds.
Don't expect to become a millionnaire unless you really go into this as a business.
Be careful about giving out personal information - photos with your pets/children could lead to unlawful incidents. If a child is modelling something for you, mask their face - you are not selling them remember! If you are having someone collect from your home, be careful. You have given your address to a stranger!
Ebay buying and selling is addictive. Enjoy but be careful. You will find me as honest as possible - but we all make mistakes. I try not to make them. ;D