These electrostatic headphones are in excellent condition, with very little use.

Currently on sale here for 999€ - https://www.audiophonics.fr/en/audio-headphones/kingsound-ks-h4-electrostatic-headphone-hifi-silver-p-10321.html

Supplied with additional 2.5m Cable worth 199€ - https://www.audiophonics.fr/en/headphones-cables/kingsound-cable-for-electrostatic-headphone-esl-ks-h4-25m-p-10326.html and compatible Dekoni Elite Velour Earpads worth 49€ - https://dekoniaudio.com/product/elite-velour-ear-pads-for-fostex-th900-headphones/

Here is a review of them being used with a Kingsound M03 portable energiser - https://headfonics.com/the-m-03-ks-h4-electrostatic-system-by-kingsound/ - https://headfonics.com/the-m-03-ks-h4-electrostatic-system-by-kingsound/ but they are compatible with Stax Pro Bias Energizers/Amps.

Specifications:

• Frequency response: 6Hz - 42kHz
• Impedance: 115kΩ to 10kHz
• Input level: 150mV (100V output)
• Maximum output level: 116dB
• Yield: 106dB at 100Vrms
• Electrostatic Capacity: 139pF
• Voltage: 550 V DC

Physical characteristics:

• Chassis Brushed Aluminum
• Anodized Silver
• Axis of the rotating structure at 90 °
• Detachable cable
• Cable length: 1m
• Supplied with a 380x260mm reinforced carrying pouch to accommodate documents, tablets or laptops
• Headset Weight: 0.270kg
• Weight of the pouch with helmet: 960g