Average cost is hiding your real resale margin
Two identical products can have very different costs. Why used-goods sellers should track margin by unit, what belongs in item cost, and how to start.
Average cost works well for a brand buying 5,000 identical units from one factory. It works badly for a reseller who paid $310 for one phone on Monday and $420 for the same model on Thursday.
In resale, the buying price is where much of the profit is made or lost. If your reports blend those prices together, you cannot tell which purchases were good ones.
Three identical phones, two different stories
Here are three used phones of the same model, with illustrative numbers.
- Using average cost
- Using each phone's actual cost
Both methods add up to $345, yet they rank the three sales in opposite order.
| Unit | Actual cost | Sold for | Margin using average cost ($365) | Actual margin |
|---|---|---|---|---|
| Phone 1 | $310 | $480 | $115 | $170 |
| Phone 2 | $365 | $470 | $105 | $105 |
| Phone 3 | $420 | $490 | $125 | $70 |
| Total | $1,095 | $1,440 | $345 | $345 |
The totals match, so the monthly report looks fine. The decisions it suggests do not. Average cost says Phone 3 was the best sale of the three. It was actually the worst. And if a buyer keeps paying $420 because the average looks healthy, the problem stays hidden until the average starts to slide.
The total can be right while every individual answer is wrong. And your decisions get made one item at a time.
What belongs in an item's cost
Purchase price is only the beginning. Anything you spend to make one specific item sellable belongs on that item.
| Cost | Example | Usually overlooked? |
|---|---|---|
| Purchase price | What you paid the seller | No |
| Inbound shipping | Freight in, or postage on a single buy | Often |
| Authentication or testing | Third-party checks, device diagnostics | Often |
| Repair and parts | New battery, sole cleaning, leather work | Very often |
| Cleaning and prep | Detailing, data wipes, re-boxing | Very often |
Then, at the point of sale, take off channel fees and outbound shipping to get the contribution from that sale.
Owned and consigned items also need separating. A consigned item's revenue is really your commission, so reports that mix the two can overstate both your sales and your margin.
How to start, even with a spreadsheet
Do this from today, and leave old stock for later. Trying to reconstruct the past is the reason most of these projects stall.
Enter the purchase price when the item arrives, while the receipt is in front of someone.
When an item goes for repair or testing, add that cost to the same unit straight away.
Record which exact unit sold, so margin comes from that unit's cost and not from a product-level figure.
After 60 days, group sold units by where you bought them. Some of your favorite sources will look worse than you remember.
How StackKnack helps
StackKnack Console keeps cost with the unit from the moment it arrives.
- Intake records each unit's cost, condition and owner.
- Orders are tied to the exact unit sold, with its cost and ownership context attached.
- Two devices of the same model stay separate, each with its own history.
- Reports are backed by an activity trail, so you can trace a number back to the items behind it.
If you also use StackKnack Pulse, those same unit records feed its daily buying and pricing recommendations, and each recommendation shows the cost, age and sales facts it was based on.
Want to see true margin by unit in your own business? Bring a few recent sales, and we will walk through how Console would record them.