The process can be straightforward.
Step 1: Identify the model
Send the complete Sennheiser model name and generation when known.
Step 2: Take clear photographs
Show ear cups, pads, headband, model label, cables, connectors, case, box and visible damage.
Step 3: Explain condition
State whether both channels work and describe battery, Bluetooth or charging behavior for wireless models.
Step 4: Provide quantity and location
For bulk inventory, send a simple list with working and defective quantities.
Step 5: Coordinate the transaction
For suitable purchases, pickup or transaction arrangements can then be discussed.