If buyer and seller are within the same state, the seller is supposed to collect sales tax and pass it on to the state. Merchant vs. individual is not a factor. Garage sales count too.
If the sale is across state lines and the seller has no presence in the buyer's state, the seller has no duty to collect sales tax, but the buyer is supposed to report the purchase to their state and pay sales tax.
Nobody would try to collect sales tax at a yard sale and nobody would fill out a form with the state to pay the $14.83 in sales tax they technically owe for the iPod they bought from Amazon. States that rely on a sales tax are quite aware of how much is being lost to these types of dealings. That's what the eBay tax thing is about. Trying to get those taxes collected that are actually owed, but are currently going unpaid.
The second tax being mentioned by mosinon is either a capital gains tax or an income tax, depending on the nature of the sale and what the sold item is. This tax has nothing to do with sales tax or with eBay or others offering to collect taxes during the transaction. The sales tax is based on the full amount of the sale, not on the sellers profit/loss. The sales tax should be handled as stated above, but the seller may owe additional taxes on the gain/profit. If the item is sold at a loss, sales tax still applies, but the lost value can be claimed as a deduction by the seller.