I wouldn't call ebay's prices "awesome", especially for current production or just off production goods ... retail prices are often cheaper than ebay, and shipping is better retail wise
In Michigan, if you sell something (goods and certain services) to someone else, you are required to collect sales tax or report the sale of goods / services to the government so they can collect sales tax from the purchaser ... in reality, this almost never happens (look at bake sales, yard sales, ebay auctions, swap meets, etc) ... your location is blank, so its hard to guess what the law says for your area ... most states in the union follow a practice of uniform code of law, so a law in one state will most likely have a counterpart in another.
be an educated consumer and double check these "great deals" before commiting to a purchase ... check ebay's "completed items" listings to find out what that same item normally ends bidding at ... and then compare that price to what you'll find online from retail stores (froogle.google.com or pricegrabber.com). also watch out on the shipping - lots of items that are listed "cheap" end up having insane s&h fees attached, often 2-3x what a retail store would charge!