Read through all the advice on Ebay/Paypal, they cover everything.
Have a Paypal account, and only take money through that. Ebay have an efficient system using Paypal, which works well.
Never ever use American Express, strange things can happen to money going through them. If they want to do unusual things like pay once it arrives, don't be persuaded.
Get a receipt and proof of posting, then if they say it has not arrived you can claim on it.
If you are selling something that people want, at a reasonable starting price (or "Buy Now" price if you are doing it that way) your relative inexperience will not matter. Check similar things already on sale to see what the starting price is and how much they go for.
You will probably find that bidding is very slow until the last few minutes, when people put in silly low bids to get it cheap, so set your starting price at a level that you are just happy to let it go at.