Tax Basics for Inventory-Based Businesses: COGS, Resale Certificates, and Sales Tax
The full breakdown — cost of goods sold, how to track purchase cost, the Wayfair online nexus rules, and how consignment is treated differently.
ReadRetail shops · Vintage resale · Online sellers · Product businessesYou're taxed on margin.
You're taxed on margin.
Not on what you sold.
When you buy things to resell, cost of goods sold reduces your taxable income before anything else. That's the fundamental difference from a service business — and it requires tracking purchase cost, managing sales tax on both sides of every transaction, and understanding a tax picture that has more moving parts than most new owners expect.
The tax picture
Three things that work differently for product businesses.
1
Cost of goods sold reduces taxable income
Every item you sell has a cost — what you paid for it. That cost is deducted from revenue before you calculate profit. A business doing $80,000 in sales with $35,000 in inventory cost pays income tax on $45,000, not $80,000. But only if the purchase cost is tracked.
How COGS works2
Resale certificates shift — not eliminate — sales tax
When you buy inventory to resell, you can use a California resale certificate (CDTFA-230) to skip sales tax at purchase. The tax doesn't disappear — it moves to your customer. You collect it when they buy, then remit it to the state. The certificate only applies to items you'll actually resell.
Resale certificates explained3
Sales tax collected from customers is not your revenue
Sales tax you collect belongs to the state — it's held in trust until you remit it. It never hits your income. The filing frequency (monthly, quarterly, or annually) is set by the CDTFA based on your sales volume. Missing a filing date triggers penalties and interest that compound quickly.
Sales tax guideExample
A vintage resale business — year one
Sales revenue
$38,000
What customers paid
Cost of goods sold
−$14,200
What you paid for items sold
Gross profit
$23,800
The taxable margin
Tax is calculated on
$23,800
Not the $38,000 top line
The remaining $4,200 of unsold inventory carries forward to next year as beginning inventory — you'll deduct that cost when those items eventually sell. This is why tracking purchase cost at the time of acquisition matters: it determines not just this year's taxable income, but next year's too.
Project your own first-year tax billCommon mistakes
The ones that cost product businesses the most.
Not tracking what you paid for each item
COGS requires knowing the cost of every item sold. Without purchase records, you're estimating — and the IRS can challenge estimated COGS. Track cost at the point of purchase, not at year-end when you're trying to reconstruct it.
Treating revenue like profit
A retail business that does $80,000 in sales might have $35,000 in inventory cost. Net profit is $45,000 — not $80,000. Setting aside taxes on revenue instead of profit means you're either over-reserving or underpaying. Run the numbers on margin, not top-line.
Using a resale certificate for everything that leaves the business account
The resale certificate exempts items you intend to resell. It doesn't apply to supplies, equipment, furniture, or anything you consume in the business. Misuse creates back taxes, interest, and potential permit consequences. When in doubt, pay the tax and check with your CPA.
Not having a seller's permit before the first sale
California requires a seller's permit before you make your first taxable sale. It's free and takes minutes through the CDTFA. Selling without one puts you behind on sales tax obligations from day one — and creates retroactive liability.
Assuming marketplace platforms handle everything
Etsy, eBay, and Poshmark collect and remit California sales tax as marketplace facilitators — but that doesn't eliminate all your filing obligations. You still need a seller's permit, may owe sales tax in other states above Wayfair thresholds, and need to track your own sales for income tax purposes.
Not building a tax reserve because 'most of the money went to inventory'
Inventory purchases are COGS, not tax-free income. You still owe income tax and SE tax on your net profit. If you reinvest all cash into more inventory, you may have a profitable business and no cash to pay the April bill. Reserve 25–30% of net profit — not revenue.
If you have a business partner
The operating agreement is the document everyone skips — until they need it.
A multi-member LLC needs an operating agreement that documents what each partner contributed, who does the day-to-day work, how profit is allocated, and what happens when someone wants out. When contributions of capital and labor aren't equal — one partner invested more money, one is running the operation — the agreement is how you protect the partner doing more of the work.
Free planning tools
Run the numbers before April runs them for you.
Tax Reserve Calculator
Calculate how much to move to your tax account on every payment, based on your net margin
Open toolFirst-Year Tax Projection
Model your full first-year tax bill — federal, SE tax, and California — before April surprises you
Open toolQuarterly Estimate Planner
Build a quarterly payment schedule and know exactly what's due and when
Open toolCalifornia Quarterly Tax Planner
California's quarterly schedule doesn't match federal — see your state payment dates
Open toolEntity Structure Analyzer
Compare sole prop, LLC, and S-corp at your profit level — see when the S-corp election saves money
Open toolOperating Agreement Checklist
If you have a business partner, answer 5 questions and get a personalized list of what your operating agreement needs
Open toolReading
The articles that matter for product businesses.
Are Resale Purchases Exempt From Sales Tax in California?
How CDTFA-230 resale certificates work, when to use one, when not to, and what misuse actually costs.
ReadSales Tax for Small Business Owners
Nexus, registration, California rates, how to remit, and the four mistakes that create liability.
ReadDoes Every Business Transaction Need to Be Accounted For?
Yes — but not every transaction is an expense. How to categorize inventory purchases, owner draws, and personal expenses that run through the business account.
ReadAccrual vs Cash Basis: Which Method Should Your Business Use?
Most small retail and resale businesses can use the cash method even with inventory. What the exception covers and how it affects your bookkeeping.
ReadHow Much Should I Save Each Month for Taxes?
Why profit — not revenue — is the right starting point for a product business, and how to build a savings habit that actually works.
ReadLLC Operating Agreement: What to Put in Writing When Two People Share a Business
If you have a partner, this is the document that protects both of you — especially when contributions of money and labor aren't equal.
ReadHow Should Business Owners Pay Themselves?
Owner draws, distributions, and S-corp payroll — how the mechanism depends on entity type and why getting it wrong creates audit risk.
ReadWork with Matt
Ready to build a plan?
Matt Reese, CPA works with retail and product-based businesses on entity structure, sales tax compliance, inventory bookkeeping setup, and the full tax picture — business and personal, coordinated.
Tax services provided through Matt Reese, CPA. This page is educational and does not constitute tax or investment advice.