Most resellers track cost basis and profit per item pretty carefully. Fewer track the number that actually predicts a cash squeeze: how much of what you bought is still sitting there. An item that hasn't sold yet doesn't show up as a loss on any spreadsheet — it just quietly holds cash you can't spend on the next haul, until one day the bins are full and the sourcing budget isn't there. Sell-through rate is the metric that catches this before it becomes a problem instead of after.
What sell-through rate actually measures
Sell-through rate = (units sold in a period ÷ units you had available to sell) × 100
The cleanest way to use it as a reseller is per lot or per category, over a fixed window: buy 25 items in a mixed lot, and six weeks later count how many of those 25 have actually sold. That fraction — not your gut sense of "most of it's gone" — is your sell-through rate for that lot at that point.
It's a different question from profit margin. A lot can have great margin on paper and a terrible sell-through rate at the same time, if the pieces that haven't sold are the ones tying up most of the money you put in.
A worked example
Say you buy a 25-item estate lot for $310. Six weeks in: 14 items sold, 11 are still in bins.
- Sell-through rate at 6 weeks: 14 ÷ 25 = 56%
Whether that's fine depends entirely on what's left. If the 11 remaining items are strong pieces still waiting for the right buyer, 56% at six weeks might be completely normal for that category. If several of them are the kind of low-value filler that comes along in every lot — the sort of stuff cost-basis-per-item accounting already tells you cost you almost nothing — then your lot's real ceiling might be something like 16 of 25, or 64%, and you're already at 88% of what this lot was ever going to sell. Sell-through rate only tells you the fraction that's moved; it doesn't tell you on its own whether what's left is slow-but-coming or just dead stock you haven't admitted to yet. You still have to look at the actual 11 items.
Why this doesn't show up in your margin math
An unsold item isn't a loss. It's just cash that used to be liquid and now isn't. Real profit margin accounting already flags "inventory holding" as a cost that doesn't appear on any line item — sell-through rate is how you actually measure it, category by category, instead of feeling vaguely like you have too much stuff.
Run it by category rather than only per lot, and it turns into an early warning system. A reseller doing well on shoes (fast sell-through) and quietly overbought on graphic tees (slow sell-through) won't see that split in a blended profit number — every sale still nets a fine-looking margin. Sell-through rate by category is what shows the tees are eating sourcing budget that isn't coming back for months.
Weeks of supply — the same idea, as a countdown
A second version of the same math answers a more direct question: how long until I need to buy more of this?
Weeks of supply = units currently on hand ÷ average units sold per week (that category)
If you're moving 3 graphic tees a week and have 40 sitting in bins, that's over 13 weeks of supply — more than three months of inventory before you'd need to source another tee even if you stopped buying today. That's the number worth checking before adding to a category, instead of "is there room in the bin."
The Q4 folklore problem
"Stock up hard for Q4, it's the big season" gets repeated so often nobody checks it against real numbers. At the national level it's smaller than the folklore suggests: per the National Retail Federation, November and December combined have averaged about 19% of total annual US retail sales over the last five years — barely above the roughly 17% share those two months represent on the calendar. The 2025 holiday season (November 1 through December 31) grew 4.1% over 2024 and topped $1 trillion in US retail spending for the first time, per NRF's Retail Monitor data — a real and reported milestone, but relative to the other ten months, still a modest bump, not the cliff "everyone buys extra for Q4" implies.
That doesn't mean seasonality isn't real for specific resale categories — it plainly is for holiday décor or winter coats, and it's closer to flat for phone cases or basic tools. What it means is that the instinct to blanket-increase buying every October isn't backed by the scale the folklore claims. What is backed is your own sell-through rate for the same categories in the same months last year, if you tracked it. Buy against your own history, not against a number nobody checked.
What this means in practice
- Compute sell-through rate per lot or per category over a real window
- (30, 60, 90 days) instead of eyeballing how full the bins look.
- Watch the trend, not one reading. A category sliding from 70% to 50%
- sell-through over a few months is the signal to slow sourcing there,
- before it turns into a bin of stock you can't move.
- Check weeks of supply before you buy more in a category you're
- already carrying — it answers "do I actually need this" in a way "it was
- a good price" doesn't.
- Don't blanket-increase Q4 buying on folklore. Check your own
- category-level sell-through from last year's Q4 first, and size this
- year's buying to what your own numbers showed, not a general "holidays
- are huge" assumption.
- A great margin on a slow-moving lot is still a cash problem. Combine
- this with the [liquidity discount already built into a sourcing
- ceiling](/blog/reseller-sourcing-max-buy-price-formula) for anything you
- already know sells slowly.
PalmFlow doesn't compute sell-through rate for you today, but it does keep per-item cost basis and status on everything you've listed, which is the raw data this formula needs — pull sold vs. still-in-stock counts by category and you have your number. Free plan, 50 items, no card.