Here's just a few things I've picked up in my experience, maybe some is obvious but when you start out then it may not be so.
My top tips for sellers
Make sure you use accurate description as much info as possible.
For clothing make sure you put as many measurements as you can so there is more chance you will get a sale, there is a good measurements guide on ebay use that as a guide for doing this. Make sure you state an item is stained otherwise you stand to get bad feedback. Don't state 'as new' if it's not and also don't sell anything you think you would not not buy yourself. Sometimes I think people use ebay to get rid of old tatt when it should have been binned or sent to charity shop.
If there is one thing that puts buyers off is a bad photo! Make sure your pictures are good quality digital or scans work well if image is dark try altering with photo programme. It should be right way up so people don't get cricked necks trying to view them!
Use picturetrail.com for additional images saves on ebay fees!
My top tips for buyers
Check feedback before buying, some negative is not a problem if they have a lot of positive feedback.
Ask as many questions as you need to before bidding so you don't have second thoughts. (especially for clothing as in my experience if you don't then you will regret it)
Don't place a bid until just before item is ending, bidding early just encourages prices to shoot up and you could have got an item for much less if you are a little patient. It's not as if you wil never have another chance to buy an item!
If you buy an item and it's not what you wanted or doesn't fit, then why not email the buyer who you outbid and see if they want it and offer to list it