Sure, if you have something you want to sell, go ahead and post. However, if it's a tangible item, you're probably going to have a lot more luck selling it on the auction house... to find a good price, look at the buyout price per unit (hover over the item and it will show you a per-unit cost.) Alternatively, you can install an addon like Auctioneer... though you will need an older version, since Auctioneer has since been updated for 3.0.2. It is very useful for determining the best prices for goods.