I've sold quite a few things on eBay (not huge amounts or anything like some people who make it their home business).
But.... I would have to say - definitely research what you are selling. Meaning, look for similar items that are for sale and that have already sold to see what they are going for. This gives you a good idea of what you may get for your items.....and, it could also make you double think about selling some things because if you were counting on getting like $100 for an item... but recent similar ones that have sold only went for $20, it might be worth it to just wait.
Be careful with shipping costs. Go to the USPS site and estimate how much it may cost you to ship (some things get real expensive depending on the size of the box), and you don't want to lose a lot of money on shipping.
Something that a lot of people on eBay do is to charge more for shipping an item because eBay doesn't collect any fees from shipping costs. They collect fees for listing and then a percentage of the selling price.
I keep thinking of things as I type this....so sorry if it gets long!!
If you know that you won't accept less than a certain amount for your item, you can set a reserve price.... which means that if the auction doesn't make it up to that price or over, you are not obligated to sell it to the highest bidder.
Another thing with shipping.... consider where you may ship to. Anything outside of the U.S. is going to cost more and there are additional regulations. Sometimes it's just easier to say in the auction that you will only ship to U.S. addresses.
I'll try to keep thinking of more things!
Denise
Yes... you still get charged the listing fees (which vary depending on what options you choose - they pretty much charge extra for everything). But, you don't get charged that percentage I was talking about on the final price.
So, they charge the listing fee based on what you set your starting price at (like if you set the starting price at one penny, it's cheaper than if you start the bidding at say 20 dollars). Then they charge you for any enhancements or extras (like if you want more pictures to add, or to bold your ad, or an extra line under your listing title, etc).
Hi, not only check to see what like items are selling for but you can also check auctions that have already ended to see what the ending price was.
Take good pictures!
Be descriptive with clothes - size, measurements, colors, etc.
Be honest. Even if there is a tear or a nick or whatever on something, let it be known. People still buy, they just want to know up front.
yeah, don't overdo it with shipping charges, that is the one thing that turns me off on an auction, instantly. I'm not paying $10 for someone to ship a book! 
(obviously trying to make money on shipping)
To help increase your listing's exposure:
-Have enough options to pay - Paypal, money order, etc. If you accept a check, make sure it's known you won't ship until it has cleared.
-Make sure you don't have misspellings in your title. People search off of those key words. You wouldn't believe how many things people misspell and then their item doesn't get much traffic.
-I usually put my titles in all caps, depending on the item/value of item, I will purchase the extras like highlighting or bolding the title.
-Setting your auction to end on a Sunday evening is usually optimal - people are home. 5-7 days listings are optimal.
Sorry this is long, I'm trying to remember important things I wrote down. A friend and I recently went through some books on auctions (specifically eBay) and the psychology of them. They helped a whole lot!
I'll come back if I think of more but I have sold quite a few things and once you do a few, you'll get the hang of it and know what to tweak.
Good luck! 
eta: Oh another biggie - don't set your initial asking price too high! You won't see much bidding action if it is too high. Don't be afraid to start low, if it's worth it, the bidding will continue to go up. People get sucked into the whole "psychology" of it all and start to take ownership of the item like it's already theirs. LOL I know I do if it's something I really want.
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