Well, for one you would have to read the fine print on the listing. If it says new/like new/no damage etc you can write and confirm the condition. If the seller responds with "there is no damage", keep the email as evidence if needed.
If upon arrival it IS damaged (even cosmetically) check out the packing to see if there is any evidence of the damage occuring during transport. BTW: buy shipping insurance if possible.
If the packing is in perfect condition and it is readily apparent that the damage was present BEFORE shipping, contact the seller and either ask for a return/refund or settle on a monetary value of the damage.
If you don't want the item and the seller will not budge or work with you, then contact the dispute console immediately and ebay/paypal will step in.
As in all transactions buyer beware. If it is too good to be true, it often is.
BTW: check out the seller's feedback, if he's 98 or 99 % positive, you can count on him wanting to have a good deal and not rip you off.
Personally I NEVER deal with any seller that is below 98% and even then I read every negative comment and if so, ask the seller about it....(in most cases the negatives are more miscommunication than anything).
Also check out his or her history: is this a one time transaction? do they sell all the time? do they have lots of other items listed? This will tell you whether a negative feedback will bother them or not. If they've only sold 2 items in 3 years, you bitching them out isn't worth a pinch of cat shit. However, if they have 100 items listed, have sold 100 items in the past, you can bet a negative comment WILL be worth something to them......afterall, if their score drops below 98%, lots of people won't bid, thereby reducing his profit or his ability to make a profit....
I compare ebay to any purchase: if you're going to spend $5000.00 on a stereo, do you buy it from bob's hifi in a tiny shop behind the 7/11 or do you buy it from someone like Bay Bloor Radio? WHile you may get a better deal at bob's you can bet BBR will stand behind the product 100% until your satisfied.
However, if you're buying a $12.00 clock radio, who gives a rat's ass where you get it because, hey, it's only 12 bucks.....if it lasts 6 months you've gotten your money's worth.....