For sale a Markins Ballhead Q-Ball M20 in great conditions (Mint to LN). The online prices I could find for new go between 340-400 US - mine goes for 250 US plus shipping! PayPal accepted (in this case, buyers will pay fees).
The ball will held 45 kg (exact specs here), controls are very logically laid out and it's very easy to use. It includes a Kirk quick release plate (knob, not lever).