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This is all directly from DMV's website:

How are vehicle sales taxes calculated?

The Nevada Sales and Use tax is actually several separate taxes which fund state and local government, schools and special purposes. See the Nevada Department of Taxation Tax Map for rates and other details. Taxes on vehicle sales vary by the type of sale and the tax rate of the county in which the vehicle is based.

Nevada Dealer Sales - Taxes are paid to the dealer based on the actual purchase price.

Out-of-State Dealer Sales - DMV will verify whatever taxes were paid and charge any difference between that amount and what would have been due in Nevada.

Private Party Sales - Taxes are paid to DMV based either on the depreciated Manufacturers Suggested Retail Price (MSRP) or a vehicle appraisal (NRS 374.112). Appraisals can be completed at the VIN Inspection Station at larger DMV offices.

If the MSRP is used, the original retail price is depreciated 15% after the first year and 5% per year thereafter. The minimum valuation for sales tax purposes is 10% of MSRP or $100, whichever is greater. The amount of depreciation must be rounded to the nearest whole multiple of $20.

Private party (or "occasional") vehicle sales are exempt from the 2% state sales tax, but are subject to all other relief, support and special taxes. See the Nevada Department of Taxation for the current rate in your county and subtract 2%.

Sample Sales Tax Calculation (Private Party Vehicle Sale)

Vehicle 2 years old with an original Manufacturers Suggested Retail Price of $12,500.

Registered in Clark County.

$12,500.00
x .80

Vehicle MSRP depreciated 20% after two years
$10,000.00
x .055

Clark County tax rate of 7.5% minus 2%

$550 Sales Tax


Reference:
Nevada Revised Statutes Chapter 374 Sections 112 and 113