(First of all i’m not an attorney and this is not legal advise.)
These type of attorneys are very aggressive. They are paid to intimidate competitors.
I had a similar experience registering a brand.
I don’t know if they already filled a lawsuit or they are just “going fishing”. If they were serious you’ll receive the legal requirement in your mailbox. Maybe they don’t have a real case behind this but they can go further.
At some point in the process you’ll have to argue your right to build a marketplace and how yours is different from vinted.
If you receive a legal requirement is time to find legal advice.
In any case you should have your legal stuff in order, terms of service, and so on.
Take this event as part of doing business. In this job is normal to deal with all kinds of BS that aren’t business. Don’t let it affect you. Really.
Good luck man.