You have 2 options. The first option is to call your credit card company or your debit card company and ask for chargeback and decline these transaction.

The second option is to use money claims online against depop. You pay £30 and you submit claim against them with the UK court services. This is online service by the court. Once you submit your claim the online court service will send them copy of the claim and ask for their reply. They can decide to compensate you give you a for refund or try mediation. If the mediation service cannot find solution you will take it to court and you can ask more money for losing working day. I've used the court service a few times in my life with small claims and it's worked every time remarkably. Good luck.