10 Critical E-commerce Mistakes and Concrete Fixes

The most expensive mistakes in e-commerce come not from missing knowledge but from deciding without measuring. The ten mistakes below recur among marketplace sellers, and most close with a single correction. What they share is that their cost appears with a delay — which is why they are hard to notice.

1. Setting prices without calculating commission

Pricing against a competitor means direct loss in a high-commission category. Commission is not a single flat rate; it varies by category and is published VAT-inclusive. Adding VAT on top produces roughly a 20% double count.

Fix: before setting a price, run a calculation that includes commission, shipping, the platform service fee and the return share. The free calculator is enough for this.

2. Selling low-priced products while forgetting fixed costs

Shipping and the service fee are fixed; as the sale price drops, their proportional weight rises. There is therefore a "floor price" below which no product leaves a profit.

Fix: if the service fee plus shipping exceeds a quarter of the sale price, sell that product as a multipack rather than a single. Raising basket value works better than cutting price.

3. Keeping separate stock per store

Showing the same 10 units as 10 in two stores is the definition of overselling. The result is cancellations, then a performance score drop, then lost visibility.

Fix: set up a shared stock pool; the one correct stock value should live in your system, not on the marketplace.

4. Ignoring barcode and SKU inconsistency

The same product carrying different codes in different stores causes the integration to update the wrong item. Products with variants need a separate code per variant.

Fix: clean the catalog before enabling automation. Any setup that skips this step multiplies the error faster.

5. Not tracking shipping deadlines

Much of marketplace seller scoring depends on on-time shipping. A single missed order costs far more than that order.

Fix: set alerts for orders approaching their shipping deadline, and sort the order list by that deadline rather than by platform.

6. Delaying customer questions

A customer who asks a question is the customer closest to buying. If the reply is late, they usually buy elsewhere. Response time is also a performance metric.

Fix: gather questions into one queue and define verified, ready answers for the ten most frequent ones.

7. Leaving out measurements

A significant share of returns are "not what I expected". The cure is not better photography but clearer dimensions and material information.

Fix: publish a table with the item's own measurements. In apparel, fit varying between brands is on its own the single largest cause of returns.

8. Leaving return costs out of the profit calculation

Outbound shipping, return shipping and handling on a returned order cannot be recovered from a product that was never sold. That cost has to be spread across the units that did sell.

Fix: add the return share per unit to your calculation: return rate × cost per return. Without this line, high-return products look systematically more profitable than they are.

9. Listing products in the wrong category

The wrong category breaks two things at once: the commission rate applied and whether the product appears in the right searches. Until noticed, it is a cost repeating on every sale.

Fix: review the category assignment of your best-selling products. A small catalog correction often pays more than price optimisation.

10. Deciding without measuring

This mistake is the common denominator of the other nine. Every improvement made without knowing which product leaves a profit, why returns arrive and where questions concentrate is guesswork.

Fix: record three things for a month — net profit per product, return reasons, customer question topics. At the end of that month, data rather than intuition sets your order of intervention.

Where to start

Do not try to fix all ten at once. Order them by this criterion: which mistake affects the most orders? Items 1, 3 and 5 usually have the highest impact, and all three can be closed in a few hours of work.

For detail, see the articles on profit and loss analysis, shared stock and order and return automation.