I find the Berlin consignment shops to be quite overpriced, but overall had some interesting bits here and there so it might be worth paying over the odds for something special iykwim. From my list:
Der Neue Schwarze, Neuzwei, The Good Store.
I don't know who stocks Ortlieb, check their site, in the city centre probably some of those hybrid hipster café/bike repair shops will stock it. Probably better to just order online, I don't think they're more expensive in GBP and if he doesn't like it, it can be returned.
Wrt Vinted apathy, I feel like everything is like investing now. Get in on the ground early and ride the wave for a bit and then the masses join and that's usually a sign of inevitable decline so time to cash out and move onto the next new thing. If you're selling Margaret Howell type stuff micro it might be worth venturing into depop, I don't know why people happily overpay on there but for whatever reason they do 🤷