Tax Basics for Inventory-Based Businesses: COGS, Resale Certificates, and Sales Tax

The core difference from a service business

A service business — a therapist, a consultant, a personal trainer — sells time and expertise. Their taxable income is roughly revenue minus operating expenses. An inventory-based business — a retail shop, a resale business, a product seller — has an additional layer: the cost of the goods they sell.

That layer is called cost of goods sold (COGS), and it is deducted from revenue before calculating gross profit. Taxable income flows from gross profit, not from revenue.

This is the fundamental advantage of an inventory-based structure for tax purposes — you are taxed on margin, not revenue. But it requires tracking cost carefully.

How cost of goods sold works

COGS is not simply “what I bought this year.” It is the cost of the specific items that sold this year. The formula:

• Beginning inventory (what you had on hand at the start of the year at cost)
• + Purchases during the year (what you paid for new inventory)
• = Goods available for sale
• − Ending inventory (what you still have on hand at year-end, at cost)
• = Cost of goods sold

Tracking purchase cost: the bookkeeping habit that matters most

Every item you intend to resell needs a cost record at the time of purchase. That means:

• What you paid (the price, not what you think it’s worth)
• When you paid it
• What the item is

For a vintage or resale business buying individual items at estate sales, markets, or thrift stores, a simple spreadsheet works. For businesses buying in bulk from vendors, the vendor invoice is your record. The discipline is doing it at the time of purchase — reconstructing costs six months later from memory is both inaccurate and stressful.

The most common bookkeeping mistake in resale businesses is tracking sales carefully and tracking purchases loosely. Both sides of the equation determine your profit.

Resale certificates: buying inventory without paying sales tax

When you buy items specifically to resell them, you may use a California resale certificate (CDTFA-230) to purchase those items without paying sales tax at the time of purchase. The sales tax obligation shifts — instead of collecting it from you when you buy, the state collects it from your customer when they buy from you.

The key rules:

• The certificate applies only to items you will resell in the ordinary course of business — not to supplies you consume, equipment you use, or personal items
• It must be issued before or at the time of purchase; you can’t apply it retroactively
• The seller must accept it in good faith and keep it on file
• Misusing a resale certificate — buying items for personal use or business use and claiming the exemption — creates back tax liability, penalties, and interest

At markets, estate sales, and informal transactions, sellers may not know what a resale certificate is or may not accept one. In those cases, you pay sales tax at purchase — but that sales tax paid becomes part of the item’s cost basis (included in COGS when the item sells).

Collecting sales tax from your customers

When you sell taxable tangible personal property in California, you are required to collect and remit California sales tax from the buyer. This applies whether you sell in person at a market or pop-up, from your own website, or through a third-party platform.

Practical considerations:

• Seller’s permit: You need a California seller’s permit from the CDTFA to collect sales tax. Many new resale businesses already have one — if not, it’s a free registration through the CDTFA website.
• Sales tax rate: California’s base rate is 7.25%, but most cities and counties add a local rate. The rate depends on where the sale occurs — for in-person sales, the location of the sale; for shipped sales, the buyer’s location.
• Filing frequency: The CDTFA assigns quarterly, annual, or more frequent filing based on your sales volume. Tax collected must be remitted on schedule — holding it too long creates interest and penalties.
• Marketplace facilitators: If you sell through platforms like Etsy, eBay, Amazon, or Poshmark, those platforms are generally required to collect and remit California sales tax on your behalf. Verify what your specific platform handles before assuming.

Online sales and multi-state sales tax (Wayfair)

Before 2018, states could only require you to collect sales tax if you had a physical presence there — a store, a warehouse, an employee. The 2018 Supreme Court decision in South Dakota v. Wayfair changed that. States can now require out-of-state sellers to collect and remit sales tax based purely on sales volume into that state — no physical presence required.

Most states have set their threshold at $100,000 in sales or 200 transactionsin the prior or current year. If you’re selling online and growing, those thresholds can be reached in states you’ve never set foot in.

For new resale businesses just starting out, this is rarely an immediate issue. As you scale — particularly through online platforms — it becomes one to track. Many platform tools (Etsy, Shopify, etc.) include sales tax reporting by state to help you monitor exposure.

Inventory businesses and accounting method

Most new small businesses use the cash method of accounting — you record income when you receive it and expenses when you pay them. Inventory-based businesses have historically been required to use the accrual method, but a small business exception applies for businesses with average annual gross receipts under $30 million over the prior three years.

For a new retail or resale business, the cash method is almost certainly available and is generally simpler. The practical implication: under the cash method, you still track inventory and calculate COGS the same way — the accounting method affects when other income and expenses are recognized, not how COGS works.

A note on vintage, secondhand, and consignment

Vintage and resale businesses often have a mix of business models — buying items outright and reselling them, selling on consignment for others, or doing both. The tax treatment differs:

• Items you own and resell: Full sale price is revenue; what you paid is COGS. Standard inventory accounting applies.
• Consignment items (you’re selling for someone else): You only report your commission or consignment percentage as income — not the full sale price. The consignor owns the item until it sells; the proceeds net of your fee belong to them.
• Items sold on your behalf (you’re the consignor): You report the proceeds as income when received from the shop. The shop’s commission is not your income — you report only your net amount.

If your business combines these models, your bookkeeping system needs to distinguish between them clearly. Mixing consignment revenue with owned-inventory revenue produces inaccurate COGS and overstated or understated income.