What happens when you can buy that item with free shipping, and you live in a state that doesn't have to pay sales tax or you find a site that doesn't charge sales tax? You math quickly goes out the window and now you get a 10% discount for a used item, not matter what condition its in its used.
The thought that someone should consider how much sales tax is on a new item from a retailer before buying a used item from a private party doesn't make sense to me - but it does to many others seemingly. It is used, it is from a private seller, someone has messed around with it, worn it, maybe lost some parts or the owners manual, came from a house where someone smokes or it has cat hair on it. Someone mentioned warranties and yes there are some items that the warranty will carry over to the next owner but there are other items that don't. If I buy an item from a retailer and have a problem with it either inside or outside of the warranty period I have a much better chance of them helping me rectify it than I ever would by asking the person I bought the item from used to help me.