Where is most of your traffic coming from now? I think how people arrive at your site makes a huge difference. For example, if they are arriving by typing in "buyer keywords" related to your niche in Google then the conversions will be higher. But if they are coming from stumbleupon or from a personal Facebook page it may be different.
You said you've sold many items. Do you have any tracking (google analytics) on your site to research what visitor paths are the most profitable? Are you just selling one kind of item and not the others?