Cost Basis Tracking for Online Resellers — 2026 Tax Guide

The 1099-K threshold in 2026 is low, and marketplaces report every reseller who crosses it. That means the IRS sees your gross sales; you need documentation to prove your net profit. Without cost basis tracking, you owe tax on the full sale price. With good tracking, you owe tax only on the profit. The difference is often thousands of dollars.

What "cost basis" actually means for resellers

Cost basis is what you paid to acquire an item, plus what you spent to get it ready to sell. For a shirt you thrifted for $4 and sold for $30, your cost basis is $4 plus your allocated overhead — the mileage to the thrift store, the box you shipped in, the label. Realistically that becomes $6-7, meaning your taxable profit is around $23, not $30.

The three cost-basis inputs

• Acquisition price. The dollar amount you paid, plus tax. Save every thrift receipt.
• Prep costs. Cleaning supplies, dry cleaning, minor repairs, materials for photos. Prorated across the batch is fine.
• Fulfillment overhead. Boxes, tape, mailer bags, labels. Track by month, divide by units shipped that month.

The four things that reduce your taxable profit

All of these are legitimate deductions but only if documented:

1. Marketplace fees — pulled automatically from your monthly statements.
2. Shipping costs — actual postage paid.
3. Mileage — IRS rate for every mile driven to source inventory or drop off shipments.
4. Home office / storage — square footage of your reselling space as % of home.

How to track without spending 3 hours a week on it

The DIY approach is a Google Sheet with one row per item. That works for the first 200 items and then breaks. What actually scales:

• Photograph the thrift receipt with your phone; timestamp = acquisition date.
• Attach the photo to the item's listing record in your reseller tool.
• Track shipping and fees automatically by connecting the marketplace API.

Instalist's Pro tier includes automatic cost-basis capture on every listing plus a year-end Schedule C-ready export. What used to take a full weekend at tax time now takes about an hour.

What most resellers get wrong

They confuse gross sales with taxable income. The 1099-K reports gross. Your Schedule C reports gross minus cost of goods sold minus expenses = taxable profit. Without cost basis documentation, you cannot deduct COGS, and you pay tax on gross. Do the tracking; owe the smaller number.

The single most valuable habit

Every time you photograph a thrift receipt on your phone, tag it with the source (Goodwill vs estate sale vs auction). At year-end, you can prove sourcing patterns to an auditor. Most reseller audits fail on documentation, not on numbers.