Are you required to pay use tax on used items?

The answer depends on the specific facts involved and the state, which I assume here is California. In general, most sales at a garage sale are exempt from sales and use tax as occasional sales. This does not however apply to vehicles, mobile homes and trailers. If the seller holds more than two garage sales a year this exemption is not valid and the seller should collect tax. If the seller does not collect sales tax in these circumstances you owe use tax. Practically speaking you would generally not report garage sale purchases for use tax. eBay purchases, on the other hand, would generally be taxable as most sellers would not meet the requirements of an occasional seller. Further, if the seller is located out of state they often do not have nexus with California and are not required to collect sales tax, but you as the purchaser are still subject to use tax. California sellers are required to separately state the amount of hte retail sales tax collected. As such, you cannot assume that the sales tax is included in the price charged.