Reselling used books is basically running an index fund (except the margins are obscene)

A few months ago, while repricing my stock on a Sunday night, a strange thought hit me: I'm doing exactly what a fund manager does. I've got hundreds of open positions, I know the "past performance" of each one, I place bets on the future, and every month a small slice of my portfolio gets liquidated while I reinvest the gains into new lines.

The more I thought about it, the more the parallel with an index fund jumped out at me. And the more I thought about it, the more I also saw exactly where that parallel falls apart. So I wanted to write this down, because I think it's a lens that changes the way you approach this business — especially when you're starting out and you reason book by book instead of reasoning portfolio.

Spoiler for the finance purists: no, it's not really an index fund. I'll get to that. But the starting intuition holds up.

The principle: betting on past performance

When you buy a share of an index fund — say an S&P 500 ETF — you're not betting on any single company. You're buying a basket of hundreds of companies, knowing that historically, the whole thing has gone up. You don't know which one will explode and which one will sink. You trust the average, and time.

Flipping used books works on the same mental reflex. When I scan a book, I look at its history: what it sold for over the last 12 months, how many times, at what frequency. And I place a bet: what sold yesterday for €80 will probably resell tomorrow for €80.

The difference is that I don't passively absorb the market average: I pick my positions from the data. And that, right there, is already not quite indexing — more on that in the last section, because it matters.

The real magic: the snowball effect

This is the heart of it, and it's where my analogy actually gets better than an index fund.

In a fund, you reinvest the dividends: nice, but we're talking 2–3% a year. In book reselling, I reinvest margins. And the margins are obscene compared to any classic financial asset.

My average margin sits around €20 per book. Not my sale price — my net margin, what's left once the book is bought and fees are deducted. On a book bought for €5. No ETF on Earth does that for you.

And that margin is what feeds the snowball:

1. I sell a book bought for €5, I collect €25.
2. I reinvest in 5 new books.
3. Statistically, I increase the number of books likely to sell.
4. So I increase my sales volume.
5. So I have more capital to reinvest.
6. Back to step 1, but bigger.

It's exactly the compounding everyone praises in investing, except here the return per cycle isn't a few percent: it's several hundred percent per winning line. The only ceiling is your ability to source stock and process it.

A concrete case: the Gabor Maté

To make this tangible, a recent and very real example.

I bought "Hold On to Your Kids" by Gabor Maté (the French edition, Retrouver son rôle de parent, Éditions de l'Homme, paperback, 402 pages) for €15. Nothing spectacular to the naked eye: a 2005 parenting book, soft cover, frankly not sexy on a flea-market shelf.

Except the data said otherwise. The sales history showed real demand, a high market price, and scarce supply. I listed it. Sold for €95 on Leboncoin, 4 days later.

€80 gross margin, on a book that 99% of people would have left in the box or dumped for €3 to a buyback platform. The difference between the two isn't luck: it's reading the right signal at the right moment. That's literally why BiblioScan exists — turning a blind bet into an informed decision is like having a research desk in your pocket.

Diversification: the winners erase the losers

Now, the honest flip side, because I hate posts that only show the pretty sales.

Not all my books are Gabor Matés. Far from it. Every month, roughly 5 to 10% of my stock sells — which means at any given moment, the vast majority of my books don't sell. Some take months. My average holding time runs around 63 days, and that average hides huge dispersion: books that go in 4 days, others that sleep for six months.

And then there are the fails. The books I thought were good that never sell. The ones whose trend collapsed. The ones I overpaid for. It's part of the game, period.

This is where the portfolio logic saves everything. Just like an index fund where a handful of companies (the Nvidias, the Apples) carry the whole index while dozens of others stagnate, my big winners erase my losers. A single book at €80 margin more than covers ten failed books at €5. You're not trying to be right every time. You're trying to be right often enough, with margins big enough, that the average comes out solidly positive.

It's exactly the mindset of the diversified investor: you accept individual losses because you're playing the performance of the whole.

One operational nuance that doesn't exist in the stock market, though: an index fund needs no maintenance. My stock does. You have to reprice periodically, track the market, drop a book that's gotten too expensive, pull the ones that have become completely irrelevant. Stock you don't touch is stock that slowly loses value.

Where the analogy breaks (and you have to say it)

I'd be dishonest to stop at the pretty picture. Because if we're rigorous, reselling used books is not an index fund. It's almost the opposite on three fundamental points, and I'd rather name them plainly.

1. It's active, not passive. The whole point of an index fund is that you do nothing. You buy, you forget, you wait 20 years. I source, I scan, I store, I ship, I reprice, I handle customer support. It's a business, not a dormant investment. If I stop working, the snowball stops.

2. I select, so I'm not indexing. An index fund refuses on principle to bet on anyone — it buys the whole market. I do the exact opposite: I sort, I choose, I bet on specific lines based on their history. Honestly, what I do looks far more like systematic investing or a value/momentum strategy than indexing. I'm a quantitative stock-picker, not a market follower.

3. Liquidity is a different planet. An ETF, you sell in one second, at market price, whenever you want. My stock sells at 5–10% a month. My capital is locked up in boxes of paper. If I need cash tomorrow, I can't snap my fingers — I depend on a buyer who wants precisely that book. It's much closer to rental real estate or watch flipping than to a liquid financial product.

What this changes in practice

If you take one thing from this reflection, it's this: stop reasoning book by book, reason portfolio.

The beginner looks at a failed book and thinks "I lost €5." The investor looks at their portfolio and thinks "my overall margin rate more than covers my fails, so I keep feeding the machine." It's a shift in focus that changes everything:

• You don't need every book to be a gem. You need the average to be good.
• You can afford the occasional blind bet, as long as diversification absorbs the risk.
• You methodically reinvest your margins instead of pulling the cash out — compounding builds the fortune, not the isolated home run.
• You accept the dispersion: some books in 4 days, others in 6 months, and that's fine as long as the portfolio keeps turning.

The only real skill, ultimately, is reading the signal. Knowing, before you buy, whether a book has real, lasting demand, or whether it's a trap. That's exactly the work I'm trying to automate with BiblioScan: replacing intuition with data, so your portfolio is made of informed bets rather than impulse buys. If you're just starting out, read how to scan books for resale first, or test a title right now with our free ISBN lookup tool.

And you, how do you think about your stock? Portfolio, or book by book? Come talk about it on the BiblioScan Discord.