When buying DVD's always keep a look out for bootlegs, especially with animes. There are some bootleg description guides for anime DVD's made by members.
If you're unsure about something, always ask questions. A responsible seller will reply promptly to questions. Always research the price of an item before buying. Lots of sellers aim to rip people off.
Keep a close eye on feedback. If the seller's percentage is less than 90%, then you should be hesitant about buying from them.
Before buying, always read the full description of an item. I've had buyers lash out at me after recieving an item, because they didn't have the common sense to read the full description. A good seller will always have a crystal clear description.
Last but not least, once you bid on an item you are agreeing to eBay's terms and conditions. Not paying could result in legal action or your account being closed.