Every reseller talks about profit.

Very few track it correctly.

The gap between what you think you earned and what you actually earned can be massive if you are not tracking every cost and calculating ROI with precision.

This guide breaks down exactly how to measure real profit, avoid the most common reseller math mistakes, and build an ROI system that scales with your business.

Why Most Resellers Track Profit Incorrectly

Many resellers assume profit is simple.

They take the sale price minus the buy cost and call it a day.

That calculation ignores:

• Marketplace fees
• Shipping label cost
• Packaging
• Taxes
• Promotions
• Refunds or returns
• Storage supplies
• Multi-unit cost averaging

Missing even one of these creates distorted ROI numbers.

Optimizing your sourcing and pricing depends on accurate math.

You cannot improve what you cannot measure.

The Correct Formula for Real Profit and ROI

You need two formulas: one for real net profit and one for ROI percentage.

Net Profit Formula

Net Profit = Sale Price minus (Buy Cost plus Fees plus Shipping plus Packaging plus Taxes)

ROI Formula

ROI Percent = (Net Profit divided by Total Cost) multiplied by 100

Where Total Cost = Buy Cost plus Fees plus Shipping plus Packaging plus Taxes

Example

• Buy Cost: 20 dollars
• Sale Price: 55 dollars
• Fees: 8 dollars
• Shipping: 6 dollars
• Packaging: 1 dollar

Net Profit = 55 minus (20 + 8 + 6 + 1) = 20 dollars

ROI = (20 divided by 35) multiplied by 100 = 57 percent

Now you know exactly how profitable this SKU truly is.

Build an ROI Tracking Sheet That Avoids Errors

A clean tracking setup prevents mistakes and gives you consistent results.

Your columns should include:

• SKU
• Product name
• Quantity
• Buy cost
• Tax
• Shipping cost to buyer
• Packaging cost
• Marketplace fees
• Sale price
• Net profit
• ROI percent
• Sell date
• Days to sell

Always calculate ROI automatically using formulas.

Never type the result manually.

This reduces errors across hundreds of SKUs.

The Nine Most Common Profit Mistakes Resellers Make

Here are the mistakes that distort numbers and create false confidence.

Mistake 1: Ignoring Marketplace Fees

Fees vary by category and can remove 10 to 20 percent of your revenue.

Always subtract them before calculating ROI.

Mistake 2: Forgetting to Include Shipping

If you charge the buyer for shipping, your costs may still exceed the label price.

Include your actual shipping cost every time.

Mistake 3: Not Accounting for Packaging

Tape, boxes, labels, bubble mailers, and inserts add up fast.

Assign a per order packaging cost.

Mistake 4: Mixing Buy Costs for Multi Unit Purchases

If you buy lots or bundles, evenly divide the total cost across all units.

Never estimate.

Mistake 5: Ignoring Returns

A few returns can crush monthly profits.

Subtract returns from your totals right away.

Mistake 6: Using Estimated Values

Estimated costs lead to estimated profits.

Use real numbers only.

Mistake 7: Not Tracking Discounts or Promotions

If you ran a sale or used promoted listings, those costs belong in your calculation.

Mistake 8: Tracking Only Gross Profit

Gross profit looks impressive, but hides true margin.

Always calculate net profit instead.

Mistake 9: Combining SKUs in Your Tracker

Track profit per SKU.

Category averages hide performance problems.

Fixing these mistakes shows your real performance for the first time.

How to Use ROI Data to Improve Sourcing Decisions

Once your ROI is accurate, you can use it to make data-backed decisions.

Identify High ROI Winners

Any SKU that consistently earns over 50 percent ROI and sells fast is a winner.

Reinvest in these products regularly.

Cut Low ROI Products

Items that deliver less than 20 percent ROI after all costs are usually not worth the time.

Reduce or eliminate them from your sourcing.

Balance High ROI and Fast Turnover

A 200 percent ROI item that takes 90 days to sell might be less profitable in practice than a 40 percent ROI item that sells every week.

Use both ROI and days to sell to judge efficiency.

Spot Hidden Cost Problems

If ROI drops across many SKUs, it may indicate rising shipping costs or high marketplace fees that require new pricing strategies.

Example ROI Comparison Table

SKU	Buy Cost	Sale Price	Fees	Shipping	Packaging	Net Profit	ROI Percent	Days to Sell
LEGO 75337	60	145	19	12	1	53	88 percent	6
FUNKO 593	10	23	3	5	1	4	28 percent	14
JOYTOY 211	38	72	8	7	1	18	38 percent	9

This instantly shows where to focus and where to pull back.

Build a Weekly Profit Review Routine

A simple weekly routine keeps your ROI accurate and actionable.

Weekly ROI checklist:

• Update all SKUs sold this week
• Verify fees and shipping costs
• Sort by ROI percent and days to sell
• Review bottom performing SKUs
• Identify winners to restock
• Review pricing strategy based on real numbers

Consistency improves accuracy and accuracy improves profit.

FAQs

Q: How often should I update my ROI tracker?

At least twice per week for accuracy. Daily is ideal.

Q: What is a healthy ROI for resellers?

Thirty to sixty percent ROI is common for retail arbitrage.

One hundred percent or more is excellent for used or collectible categories.

Q: Should I include gas or travel time in my ROI?

Optional but helpful. You can track it separately to understand true sourcing cost.

Q: What if an item has high ROI but takes too long to sell?

Use profit per day to judge overall efficiency. Slow items hurt cash flow.

Actionable Takeaways

✅ Track net profit, not gross profit

✅ Include fees, shipping, packaging, and taxes in total cost

✅ Calculate ROI automatically with formulas

✅ Review ROI weekly to guide sourcing decisions

✅ Reinvest in proven winners with fast turnover

✅ Cut low ROI products before they drain your resources

✅ Use tools as you scale to prevent data drift

Correct ROI tracking is not optional.

It is the foundation of sustainable, scalable reseller profit.

When your numbers are accurate, your decisions become smarter, your sourcing improves, and your business becomes truly profitable.