Idaho charges 6% sales tax on the full price of a used car bought from a private seller, and the county assessor's motor vehicle office or ITD collects it when you apply for the title. Unlike a dealer purchase, a private sale gets no trade-in allowance at all. Family sales are tax-free only between parents and children, grandparents and grandchildren, or brothers and sisters, and only if that relative paid tax when they bought the car. File within 30 days of the sale or you'll owe a $20 late penalty on top. All of this was current as of Oct 19, 2026.
Most of this comes from the Idaho State Tax Commission's motor vehicle pages and Form ST-133, plus ITD's title page and the statutes, all read on Oct 19, 2026. Fair warning: the commission's exemptions page says it was last updated in 2022, and the ST-133 on file is dated July 2023, so if you're claiming an exemption, check the current form at the counter. Our Idaho title transfer guide walks through the paperwork. This post is the tax.
The math is simple, mostly
Idaho Code 63-3619 sets the sales tax at 6% of the sales price. The commission's motor vehicle pages describe private-sale tax on the total price and don't mention a local add-on, so an $8,000 car means $480 in tax. Both you and the seller have to sign the bill of sale, and the office wants the original as proof of the price. A photocopy or a phone photo isn't what they ask for.
A low number on a bill of sale for a car that's obviously worth more will draw questions. The commission can tax on fair market value in some situations, including a business bringing a vehicle in from another state.
No trade-in break in a private sale
This catches people who are used to buying from dealers. The Tax Commission says trade-ins only apply when you buy from a retailer, and it says flatly that there are no trade-in allowances in private party sales. If you hand a private seller your old car plus $3,000 for theirs, that's a barter. The commission says whoever receives a motor vehicle in a barter owes tax on the full value of that vehicle. So in a straight swap between two people, both sides can end up owing tax.
The family exemption is narrow
You can skip the tax on a car bought from a family member only if that person is your parent or child, grandparent or grandchild, or brother or sister, and they paid sales or use tax when they bought it. Aunts, uncles, cousins and in-laws aren't on the list. You prove it with Form ST-133 when you register.
The form adds a few details. The family exemption covers motor vehicles only, not boats, ATVs, UTVs, off-road motorcycles, snowmobiles or trailers. A car that's community property can be sold to a qualifying relative of either spouse.
Gifts and "take over the payments"
A true gift can be exempt, but the commission lists conditions: the giver gets nothing of value back, the relationship supports calling it a gift, the recipient doesn't take on the giver's debts, and neither person owes money on the car. The giver and recipient both sign Form ST-133GT.
The commission calls out one specific case: if you let someone "take over payments" on your car, they've assumed a debt, and it isn't a gift. That's a taxable sale, even between relatives. If you buy a van from a neighbor to donate, you still pay tax when you title it in your name.
Buying out of state, or moving here
If you live in Idaho and buy a car in another state, Idaho presumes it's for use here. Tax you properly paid to the other state counts against what you owe, if you have the receipt. The commission also warns that if the other state offers an exemption for out-of-state buyers and you qualify, use it, or you'll pay tax you didn't owe there and still owe Idaho. Under Idaho Code 49-504A, the 30-day filing clock on a car acquired in another state starts when it first enters Idaho.
New residents don't owe Idaho tax on a vehicle they owned or acquired more than 90 days before moving. Own it less than 90 days and Idaho presumes you bought it for use here. Active military have their own version of that rule.
Deadlines and fees
For a private sale with no lienholder, ITD says to file with a county assessor's motor vehicle office within 30 days of the transfer to avoid a $20 late-filing penalty. The state title fee is $14, plus a county administrative fee that varies. If a lender is involved, Idaho financial institutions have to collect the tax when the seller didn't.
Check the car before you file
Look up the title brand history on NMVTIS and get a CARFAX report or AutoCheck before money changes hands. A salvage history changes what the car is worth, and our Idaho salvage and rebuilt title rules explain the brands. Shopping across the border? Our Oregon and Washington guides cover those states, and the used car sales tax overview covers the rest.
About our pricing: CarVins resells CARFAX and AutoCheck as a licensed reseller and has no connection to either brand or to Idaho's government. As of mid October 2026, it's about $5.50 for a guest report, roughly $4.50 for members, and about $6.75 for the pair, where retail is around $39.99 to $44.99. Confirm before you pay, since checkout has the final number.
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