According to the DAC7 directive, platform operators (in this case, auction houses) are legally required to report the activity of sellers who:
- Sell more than 30 items (unique item IDs) in a calendar year, or
- Receive more than 2,000 Euro in total from sales during the same period (paid out).
If your sales activity meets either of these thresholds, the auction house must report the details to the relevant tax authorities.