Before any purchase, check out the online trader in detail.
Use customer feedback (reviews) to build a picture of the seller. You can find this feedback by searching the name of the webshop plus "reviews".
Alarm bells should ring if an offer is obviously too cheap.
Check whether the seller has an imprint and whether this is complete and plausible. For example, does the seller have a plausible contact address, a correct telephone number or email address and a commercial register number?
Check the information on revocation rights, returning goods and postage costs.
Wherever possible, avoid providing a credit card number. If this is not possible, make sure that the information is only entered on an encrypted website (i.e. the browser's address line contains "https").
Carefully check your credit card bill and contact your credit card company to query any inconsistencies.