Online involves shipping cost which will drive up the price massively, especially if you're buying full towers which are expensive to ship. A computer that might otherwise be free will suddenly cost $50 because that's how much it costs to ship to you. There's also a larger pool of customers to appeal to online, so that can drive prices up further as well.
Try to find local deals that you can pick up to avoid unnecessary shipping costs.
What's your budget and what do you want to do with the PC?