Of course you can also buy used desk chairs for boys or girls at ebay-kleinanzeigen.de, quoka.de or Facebook Marketplace.
Check the wear and tear on the upholstery and the stability of the legs or the base. In addition, make sure that all the castors are in perfect condition and that the adjustment of the seat height and backrest works well.
Since buying second-hand has several risks and is not suitable for everyone, you should consider buying a new desk chair before you buy. (For example, wear and tear on the joints, breakages, incorrect fit)
Conclusion: Plain new models or swivel chairs are available for between €30-50 and you can usually return them within 14 days.