i would say the three most popular ways to sell to sell right now are ebay, amazon and craigslist.
both amazon and ebay have fees which are normally a percentage of sales while craigslist is free but normally local business.
businesses on ebay and amazon pass the fees along to customers by raising their price to compensate for this. i've seen prices for the same item vary on a sellers personal website, on amazon and on ebay.
ebay also at times offers free listings or for a set price. the most recent was for 3-4 days and up to 1000 items. not a bad deal.