Every reseller has a death pile. The tubs of stock that were going to get listed, the listings that were going to sell, the box from the estate sale that seemed like a good idea in the car park.
The pile is not the problem. Not knowing what it costs is the problem.
Turnover, in one number
Inventory turnover is how many times your stock sells through in a period. The simple version for an eBay store:
Turnover = items sold in the period Γ· average items in stock
If you carry 400 items on average and sold 300 in 90 days, turnover is 0.75 for the quarter, or 3 times a year. The average item sits four months.
Sell-through rate is the same idea seen from the listings side: sales in the period divided by active listings. A store turning at one sale a day per 100 listings is at the pace most general resale stock should run. Under that, stock is accumulating faster than it leaves.
Neither number is profit. Both tell you how fast cash comes back.
What the pile actually costs
Feelings are not useful here. Cash is.
Take a store with 600 active listings and 200 of them over 90 days old. If the average cost of goods is $8, that is $1,600 of your money sitting in the garage. If the average listed price is $28, that is $5,600 of revenue that has not happened.
That $1,600 is not a loss yet. It is a loan you made to your own shelves, at zero interest, with no due date. Every sourcing trip you cannot afford this month is that loan being called.
Add the second cost: eBay's search reads a store full of old listings as a store nobody buys from, and ranks every listing in it a little lower. The pile drags on the stock that would have sold.
Why piles form
Three reasons, and they are the same three every time.
Sourcing outruns listing. Buying is fun, listing is work. The tubs fill faster than the queue empties. This is the unlisted pile.
Listings outrun buyers. Stock gets listed, priced at hope, and left. Nobody reviews it at day 60 because there is new stock to list. This is the stale pile.
Nobody counts. Neither pile has a number on it, so neither pile is urgent. It is the only inventory problem that gets worse by being ignored, and it gets ignored because it is not visible.