1. Check your seller's reputation and background - When did they join eBay? what's their feedback percentage? If they have a negative feedback - what did they get it for?
2. Make sure the seller ships to the area where you live - once you've won an item you're committed to buy it so be very careful.
3. Check what other items the seller is selling - Is the item you're interested in buying in their area of expertise or is this a random sell making it a little risky.
4. Research before buying- A good way to do so is via eBay's advanced search (picture 1 below) --> enter the search title and description and mark the "show completed listings only" box (picture 2). You will then see a list of all completed listings of the item in the past 15 days (picture 3 below). Prices marked in green are items that where sold (red means the auction ended without anyone buying the item). Browse down the list and get a sense of the average price.
5. Communicate with the seller prior bidding regarding any questions you have. A seller that doesn't answer should make you think twice whether or not to do business with. Nonetheless take into mind the seller may have a reason for not answering (such as a personal problem) so do give them a fair chance.

Congratulations! you are now ready to start bidding.

P.S. I recommend paying as soon as you win the item- sellers appreciate this and will probably give you a positive feedback in return.