Buying a car off ebay can be simple. It can also be a right nightmare sometimes.
That quality of the car generally depends on the budget you have set yourself. If you are buying from ebay becuase you think you will get a good bargain, don't expect to get a perfect car. If you are going on ebay as an alternative to say, autotrader, and are willing to pay book price, if not slightly over, for a good car, then you will get a good car.
As said already, if you can view the car beforehand, do! If you don't view and bid regardless, you are obliged, by law, to buy the car, regardless of the condition it is in when you turn up. Obviously, there is nothing, really, to stop you from walking away if the car is not as described, although don't expect the buyer to be best pleased if you want to go over the car with a fine tooth combe before handing over the money. Don't even expect a test drive. As far as the seller, ebay and everyone else is concerned, you have bought the car, agreed a price and are just coming to hand over the cash and pick it up.
As far as payment, that is down to the seller. There are no set rules. Some (like myself) always ask for a £200 deposit via paypal on the end of auction, regardless how much the car sold for. I demand the deposit before agreeing to any collection times.
Some sellers do similar, others aren't so bothered, but you will find that most will, so be prepared to loose your deposit if you do walk away.
Other than that, just be careful. As said already, send an email with a big long list of questions regarding interior condition, exterior condition, engine service history & condition etc. If they give you an half arsed reply with questions missed off, don't bid. If they can't be bothered to upsell the car to you, they are probably trying to hide something. If they do answer all the questions and all are satifactory, go and have a look! If you really can't get to see the car, ask some more questions and then make a bid if you are 100% happy.
Don't make quick decisions in the last few mins. Set your maximum, and if it goes for more, dont worry, there will be another one. Don't get your hopes up of winning any car (unless it has a buy now) as you will just get attatched to the idea and be willing to pay over the top for it.
Anyway, just have fun I suppose. I have had some excellent cars off ebay, a number I havent viewed. But then again, I have probably had far more lemons.
My final advice would be to use ebay as a last resort. Always use Autotrader, pistonheads, classifieds etc. before resorting a bidding wars on ebay. Ebay is great for picking up sub £500 bangers, but don't be fooled into thinking they will be anything else but a banger. As with anything in life, if you want a goodun, youve got the pay for it!