GarageSale is just plane cheaper ($24.99) and won 2005 best new comer award from eBay. I'd weigh the features closely before you purchased either. I think iSale is very simple, but there are a lot of features in GarageSale.
Cool, to tell you the truth, I've never used either. I don't sell much on ebay, so I'd rather just use their online lister. Just posted this because I figured if someone's going to buy it...they mine as well save some $$$!