I have found that if you want to buy a buy it now item and there is also a bid now option, if you still bid below the buy it now price, almost everyone will offer you a second chance at that item for the price you bid on it.
Also if you want a discount on a item or for the seller to send it express instead of normal mail. A good idea it to email the buyer before hand and ask him/her, they will almost always give you a discount as you haven't bid on the item yet and they want you to be thier customer.
For expensive items always ask for more photos and any other detail you can think of. This will save you from being disappointed.