I've been selling on eBay for about 4.5 years now. I started slow with a goal of making $50 per week. When you start it will seem overwhelming. It will seem like the whole process is a lot of work (finding items, taking pictures, writing descriptions, researching value, emailing customers at the end of the auction, finding out how much shipping cost, packing, mailing, leaving feedback, following up on shipping dates to your buyer, keeping records, etc.). As you get into it you will find ways to streamline the process and what used to take you hours to do will be simplified down to minutes. When I first started I could handle 5-10 auctions at a time (part time) and now I'm still part time and have a 3 year old at home in the mix and had 119 auctions end last Sunday.
The best advice I can give is look around your house and sell off things you don't need. If you don't already have feedback built up on eBay you should establish some before you get started by buying some items or by selling off low-cost items. People will take a gamble on a new seller if they are selling low-cost items, but trying to sell something that has great value with no reputation will bring you only a low bid or no bid at all.
Sell what you know or things you are willing to take a gamble on. Keep your buyer in mind as well. Electronics, video games, movies and things that are generally sold to a younger buyer, have the highest problem rates plus the risk of damage to electronics while shipping is also greater. I have heard sellers complain about non-paying bidder, slow payers or just plain problem buyers when they are selling these kinds of items, unless they are vintage/collectible.
I sell a lot of antiques and collectible type items that I find at estate sales, garage sales and thrift stores. I have a personal rule that I won't pay more than $5 for anything I don't know. That way the worst I am out on anything if it doesn't sell is $5. It's also fun to buy something for 10 cents and have it sell for a lot more (I sold a 1964 DAM Thing...troll with a tail...for $144.50).
Packing supplies can be found very inexpensively in bulk on eBay. The same bubble mailers that can cost $0.80 each at a brick and mortor store can be bought by the case (100-200 depending on the size) for $0.10 - $0.35 each including the shipping costs. The best way to find them is to do an advanced search, put in specifications that you want a seller that's, for example, within 100 miles of you to buy your supplies from. That will reduce shipping costs as well. The post office provides priority mail boxes and labels free of charge delivered directly to your door. They can be ordered on the usps.com website. I almost always use the free boxes or bubble mailers. Oftentimes priority mail is less than fifty cents more than parcel post and you always have a new, clean box and it arrives quickly which makes you look good as a seller.
Learning the forms of available shipping is also important. Books, CD's, patterns, cassettes, DVD's and VHS tapes can ship via media mail which reduces the shipping costs ($1.42 up to a pound, $1.84 up to two pounds and so on). It is a slower form of shipping but buyers that are willing to wait are usually very happy to have media mail as an option. Large, heavy items can probably ship for less via FedEx or UPS and having them pack it is also an option.
You will have to decide what forms of payment to accept. PayPal is widely used by buyers. It's a turnoff to sellers because each transaction is charged $0.30 plus a percentage of the sale. I get around 80-90% of my payments via PayPal so the added cost is worth the extra bidders, I think. I also accept money orders and cashier's checks.
Make sure you have very clear terms of sale including how often you ship, how quickly you expect payments to arrive, what types of payments you accept and anything else you find applicable.
Be friendly and courteous in all your emails, even if someone acts like a jerk. Killing them with kindness is the best way to customer satisfaction.
One thing that I can't emphasise enough is to be available to answer questions on your auction. Answer any question as soon as it is possible. Chances are, if you don't answer questions quickly there's another seller out there that has the same item that will.
All items need a complete description. If applicable, give the dimensions of the piece or measurments (if you are selling clothing, to many buyers this is a must). Sometimes a picture does not tell 1,000 words especially if the image host is down, the picture is not clear or the lighting is not good. Describing the colors and dimensions is a good way around these potential problems. Sometimes placing an item next to the item to be sold is a good way to get perspective (like placing a dime next to an old button).
Yes, you will stress when you start and will be fearful of the item arriving and the buyer not liking it, that's part of the selling process. Last Sunday I sold $175 worth of vintage doll sewing patterns to a woman and worried all week that she would be unhappy. Thankfully she left me positive feedback last night. What I'm saying is that the stresses of potentially unhappy customers does not go away.
eBay has a selling walk-through on their site and their selling form will not let you forget anything. If you plan to do this full-time, though, I highly recommend getting an image hosting/auction listing service. I have tried a few others and now use Infrog. With an auction listing service you can set up templates so that all your terms of sale, forms of shipping, etc., default when you are writing your listing. This will save a lot of time. Also, eBay gives a seller just one free picture per listing. You have to pay $0.15 extra per picture, I believe. If you have an image host/auction lister then the additional pictures are free. Also eBay charges for auction scheduling ($0.10 per auction). Basically what that means is that you can work on the listings now and have them start at another time. I find that auctions that end in the evening do better than those that end during the day so I schedule all of mine to start in the evening. This way I can work during the day and have them start later. With my auction lister I don't get the additional scheduling fees.
Hmmm...what else...? I can't think of anything else right now. What else would you like to know?