The first thing you need to do is RESEARCH what you have. Get all the information you can about what you are selling, what it's worth, how old it is, and anything else you can find - including what similar items are selling for on Amazon, Ebay and other sites...
Second, make sure you have good quality pictures to post! The better the photos & the more detail they show the better.
Third, decide how little you are willing to take for something and set up a reserve if need be (this means if bids don't reach your reserve you keep the item). It costs money to do this, as it does to list the item, but it has saved me a few times because I've been able to get more elsewhere.
Fourth, Write up a product description & make it good... You want someone reading to think they NEED this item, but you also want it to have all the details...
I suggest you post for 3 days, that's worked best for me, but other durations can work too.
at 5:50 PM on Nov. 13, 2010