If you're talking about Ebay, auction listings sometimes have an option to "Buy It Now"...meaning that the buyer will not bid at auction, but pay the stated price. Many sellers sell only with a fixed price and don't even list at auction at all. When I started as a seller I played with auctions but it became clear to me that consistent and repeatable sales were the only way to make a living at this. I focused on products that would sell in volume. I was fortunate to own my inventory, but for those who don't have their own inventory, drop shipping is the way to go. Your margin will be slimmer - because you don't own the inventory - but if you do enough volume, you'll get to full time income.
I hope that this help. Let me know if you have more questions - and please check out my company!
1st American Pleasures