Can't help you with percentages, but can offer some advice.
First look up the item that you are selling and put a watch this item on it and see if it sold. No sense selling something that no one is not buying.
Yes there is a small charge if the item does not sell. That will depend on how many photos you list etc. E-Bay will let you knopw what the charges will be and you can back out if you want.
I have shipped a saddle across the country for $60 so think small. The PO has small boxes that are $7.95 postage paid. If the item will fit in a padded envelope - that is even cheaper. Books and Dvd's can go media mail and that saves money too.
Be sure you say which payment types you accept. Yes Paypal costs money but it is convenient for me as a buyer or seller. Beware the "low baller" who might ask you to take half of what your item is worth. One offered me $25 for something I later sold on E-Bay for $60. Also I state I will not ship overseas and that gets rid of some scammers. Beware of anyone who offers you a cashiers check for more than the amount - there are fake cashiers checks and money orders out there.
Often the person says they will send an agent to pick the item up. All they want is you to cash the check and send them money. Scam scam..
I had a lady once say an one of a pair of items was broken and she would not have paid so much. I came back with - take the item to the post office because I carried insurance on it. Well she did not want to do that. Insurance is cheaper than worrying about something that is lost or missing.
All for now, Kathy

Thanks, I totally forgot about insurance and agree about the overseas thing. I'll watch the cashiers check thing too; can I just do paypal only? See, there's so much to know. Hopefully, I won't get in over my head. Thanks again.

Yes I think you can do Paypal only, but that could cut off some buyers. If you are selling a big ticket item and someone asks to write a check just do not ship until the check clears.

I've been buying & selling about 7 yrs, and still learning. Check out ebay university, I plan to check it myself.

Here is a link that might be useful: ebay University learning center