The Money Quietly Leaking Out of Your Taxes as Sales Grow — 3 Things Online Sellers Miss

OPUS 세무·행정팀 · Tax & Administrative Support · Published 2026-07-14

Key takeaway: In online retail, the tax rules change wholesale the moment revenue crosses certain thresholds. Miss the change and you either pay tax you didn't owe or get hit with penalties later. The three things sellers most often miss: (1) the VAT status switch from simplified to standard taxpayer, (2) input VAT credits, and (3) bookkeeping and deductible expenses for income tax.

We focus on selling well. Taxes get crammed into May and July, on the assumption that the accountant will handle it. But for online sellers, the calculation method itself changes the moment your revenue band shifts. The problem is that the change arrives quietly. You cross a threshold without a single notice, and if you meet the next filing unprepared, the difference comes straight out of your pocket.

1. Cross this line and the rules change — simplified to standard VAT

핵심: If prior-year revenue exceeds KRW 104 million (~$78K), you're automatically converted to standard VAT taxpayer status on July 1 of the following year. Tax invoice obligations and the VAT calculation method both change at once.

VAT-registered businesses split into simplified and standard taxpayers, and the dividing line is prior-year revenue (gross supply value).

• Under KRW 48 million (~$36K) per year: you're a simplified taxpayer and VAT payment is waived (you must still file — skipping it triggers penalties). In this band you also cannot issue tax invoices.
• KRW 48 million to KRW 104 million (~$36K–$78K) per year: still a simplified taxpayer, but you pay VAT and take on tax invoice obligations.
• KRW 104 million (~$78K) or more per year: you're automatically converted to standard taxpayer status on July 1 of the following year.

As a standard taxpayer, filings go from once a year to twice a year (plus interim assessments in April and October), and VAT is calculated as "output VAT minus input VAT." The conversion itself isn't a loss — in fact, point 2 below shows where it works in your favor. The loss comes from not knowing and not preparing: losing clients because you can't issue tax invoices, or having cash flow shaken by a suddenly larger payment.

📌 From 2026, having your place of business in a designated "excluded area" can convert you to standard taxpayer status regardless of revenue. Check whether your business address falls under the excluded-area notice.

2. Are you paying VAT you don't owe? — input VAT credits

핵심: Standard taxpayers credit the full VAT paid on purchases (simplified taxpayers get only 0.5%). Keep documentation for VAT on platform fees, advertising, and inventory, and it comes straight off your tax bill.

Simplified taxpayers get a credit of just 0.5% of purchases, while standard taxpayers credit the entire VAT charged on their purchases. And a seller's monthly spending is loaded with VAT — platform sales commissions, advertising, inventory purchases, packaging and supplies, rent. Collect the documentation and it comes right off what you owe.

The trap is habit. A seller who never bothered with receipts as a simplified taxpayer because "it barely adds up" carries that habit into standard taxpayer status — and pays VAT they don't owe, every single filing. Three rules are enough.

• Register a business credit card with Hometax. Business-related purchases on registered cards are tallied automatically, which makes claiming credits easy.
• Get qualifying documentation (tax invoices, cash receipts). You can get tax invoices for platform commissions too.
• Don't mix in personal cards. Personal spending isn't creditable, and mixing the two can trigger a request for substantiation.

3. File without books and you can't deduct what you actually spent — income tax

핵심: File on standard expense ratios alone, without books, and you cannot deduct the gap even when your actual spending on inventory, commissions, and advertising exceeds the ratio. At higher revenue, a no-bookkeeping penalty (20% of calculated tax) is added on top.

Get past VAT and the May income tax filing is waiting. Here too, the rules change as revenue grows.

• If prior-year revenue is under KRW 300 million (~$225K) for wholesale/retail, "simplified bookkeeping" (household-ledger level) is enough; at KRW 300 million or more, double-entry bookkeeping is mandatory. (If you manufacture what you sell, you may be classified as manufacturing, which drops the threshold to KRW 150 million (~$113K) — one industry classification makes the difference.)
• File on the National Tax Service's standard expense ratio alone, without books (estimated assessment), and you cannot deduct the gap even when your actual spending on inventory, commissions, and advertising exceeds the ratio. You can't subtract what you spent, so your income is inflated and your tax goes up.
• If prior-year revenue was KRW 48 million (~$36K) or more and you file without books, a no-bookkeeping penalty of 20% of calculated tax is added. If you're required to keep double-entry books and file without them, it's treated as a failure to file at all — considerably heavier.
• Conversely, if you're only required to keep simplified books but file with double-entry bookkeeping, you get a bookkeeping tax credit (20% of calculated tax, up to KRW 1 million / ~$750).

A seller's deductible expenses add up faster than you'd think — cost of goods, platform commissions, advertising, packaging and shipping, rent, telecoms, and business vehicles (if you keep a mileage log). Actually claiming them requires records. Transacting through a business card and business account and keeping documentation for five years is what cuts your May tax bill the most.

The common thread

All three have one thing in common: each takes only a short burst of preparation at the moment your revenue grows. Miss that window and money leaks quietly, with nobody there to tell you. Taxes aren't the opposite of selling — they're how you keep what selling well earned you.

📌 If you want to stay on top of complicated tax and administrative work without missing anything, take a look at OPUS's tax and administrative support.