Damn it, this is indeed very annoying, stay strong!
Thinking of a worst-case scenario, be careful about what you share here and the way you express things: this could come to bite you as this thread is online and public. Nothing bad yet, just a first thought!
Yes, you are allowed to use the Community Edition of Sharetribe Go, as long as you use it within the license terms. We don’t use any code coming from Vinted and have no relationship with them. I don’t think Sharetribe and the technical solution you’ve picked can be involved in any way. But if this is what their claim is about (the Sharetribe Go Community Edition code), they should contact Sharetribe, not you.
I think you should first double-check that this is legit, and not a scam. Is that attorney a real one and are they really working for Vinted? I think you should contact Vinted directly to confirm that they are aware of this.
Then, I would look in detail into what they claim has been copied. Are you using some of the same content (maybe you’ve copied a few sentences for your About page, or some copy texts like some rather unique buttons?), or use very similar colors (your main color, #59a5b3, is indeed quite close to Vinted’s, #09b1ba), or have a domain name that is a copy, or have a similar logo (they look close). Or maybe in the promotion you’ve made you wrote things like “Vinted but cheaper” or whatever. Just a guess.
If you can prove you picked your color and logo before their color and logo was up, it would be obvious that if anything, they copied you Otherwise, you’ll have to defend that you didn’t know and ended up with a style that is just rather similar, or you’ll end up in discussions like “You copied the color”, “Your website looks similar to ours”, etc. I’m not sure what the legal basis for that, I don’t have any legal training so really can’t say.
One more thing to check: on your website, do you have images that are copied from their website? That could be something they are enquiring about.
Finally, it’s time to make sure all your terms and policies are up-to-date and notify your users if you’ve had holes in there.
There really isn’t much we, at Sharetribe, can do. But hopefully, other people in this community will share their tips, experiences, and contact! And of course, discussing politely with them (through email, to have some written proofs, versus telephone) can already go a long way.
If you confirm that this is a legit request they’ve sent you, it would be a good idea to seek professional advice to learn how serious their claim of “your website looks so much like ours that visitors can be confused and tricked into believing that you belong to the Vinted network” (I’m guessing/interpreting their claim) is.
Keep us posted!