When it comes to online auction sites, eBay is king. I have made around 200 transactions through eBay and have had a positive experience that far. You can just about find anything imaginable on eBay from popular new items like unlocked smart phones to a million dollar mansion on your very own island.
If you are a first timer eBayer, I would suggest bidding on a small inexpensive item, so that you can get used to the process start to finish (bidding, paying, and giving feedback). This process will hopefully also boost your own eBay identity through feedback, giving buyers and sellers more confidence about what kind of eBayer you are.
There are a few points that one should keep in mind before buying and selling on eBay.
First, I would be wary of or avoid international sellers (sellers that are outside of the US), until you are a more seasoned eBayer. There are many ins and outs to international shipping costs, duties, taxes and other details that can make overseas transactions a hassle. I think for your first purchase you should shoot for a smooth transaction and stick with an item that is currently residing in your own country.
Second, you should really know about the item that you are bidding on (or selling for that matter), and make sure that the item in the auction is indeed the same. A little research online will save you a lot of heads in the long run when you have any doubts. By searching the make and model of items and any other important information about the items you are selling or buying, you will reduce the risk of buying something you did not want in the first place. If you are still not sure you can always contact the seller directly and ask them any questions (usually in regards to condition, size, and age).
Thirdly, check out the seller's feedback and comments made from past customers. The feedback and comments section really tells you what you can expect if you should buy something from that particular individual. Also, do not forget to check out the shipping costs, since you will be paying what is posted on the item page. Many eBayers will pump up the shipping costs on reliably cheap items to increase their profit, which are in some cases way out of line.
Lastly, have fun. There are very few items that I have not found on eBay. It is a truly wonderful resource for saving money, collecting, and even making money.