I have been selling on eBay for a couple of years. I've done a little on Yahoo..but since they have so few buyers, even though their fees are much lower (neither they or eBay's listing fees are on a percentage..they both go by the amount you start your item..and Yaho's is less than eBay's), and since Yahoo has very few buyers, it's no savings..and what they do buy, goes for much less than I get on eBay...and several times, for some reason, on Yahoo..bidders don't seem to think they actually have to pay for what they bid on.
You need to start at the minimum you wil be happy getting, since that may be what the item will end at. To discover what you might expect to get, look at completed auctions. If you are selling books, or computer media, look into shipping media rate (so your buyers will still be interested, one they figure their shipping). Obviously, if your item is only worth $1.00..by the time you list it, pay listing fees, and end value fees, it's not wise to list that item. I however, sell "gadgets" for as low as $2.49 and make a nice profit. However, since I have duplicates, I can sell a Dutch auction, and often sell 3 at each listing, which costs me .30, plus .25 for a gallery picture, and 2.2% end auction fee. That still leaves me over $6.00 profit. The buyer pays for shipping. I charge $1.00 for shipping and handling for up to 4 "gadgets", and that covers the cost of the mailer as well. Of course I also sell more expensive items, as I wouldn't want to make trips to the PO for one "gadgets" order..my handling fee wouldn't cover the gas.
Now, to keep this on Disney topic LOL..we do use our auction money for our trips..already have our air and our Mother's Day meal to Cape May paid for!