I've bought quite a few electronic items on ebay - mostly obsolete video game cartridges and that sort of thing. Most of the transactions went very well.
As others have said, be sure to check feedback before bidding. Stay away from people who have had considerable negative feedback and from those who haven't been on ebay long enough to get much feedback.
In case of a problem, familiarize yourself with both ebay and paypal's systems for reporting people, and especially with the timeframe that you would have to report things in. Chances are that you won't have to report someone if you are careful about who you buy from, but it's a good idea to at least know how.
Before you bid on something, email the person and ask them about their policy if the item doesn't work when it arrives or if there is some other problem. Some people give full refunds, others do partial refunds, others require you to send the item back, etc. Also, ask the person what shape the item is in, when they last tested it to see if it works, etc. It's best to know before you bid. It's also important to know how much you will be paying for shipping before you bid.
Lastly, make sure that you know how you are going to pay for the item. Paypal, cheque, money order etc. Not every seller accepts every payment method, so be aware of what they will and won't take.