It is a known scam within Germany, see (e.g.?) [here](https://www.watchlist-internet.at/news/vorsicht-wenn-die-wohnungsbesichtigung-ueber-bookingcom-abgewickelt-werden-sollte/).

Relevant points

- "landlord" claims he is living abroad
- "landlord" claims he had trouble with potential tenant before
- "landlord" wants you to place deposit at booking.com