The problem with eBay is that the fees you pay as a seller after the transaction is complete can be significant, especially with higher priced items. A few years ago, the cut eBay got was a lot less... I remember paying reasonable fees for selling expensive items, but no longer. I recently sold a digital SLR camera on eBay, and between the insertion, final value, and Paypal fees, it ate up nearly 20% of the selling price. And, add to that any Paypal fees that are charged when the seller sends you money (remember, eBay owns Paypal!). And forget about it if the seller complains about the item, delivery issues, etc.
That's why I use Gazelle... they offer you a reasonable price with no hassles. For me, lack of aggravation and my time are worth it. But, to each his own...