A buyer can counter the seller's offer from the vehicle lot page. You have three options available to you: "Increase Your Bid", "Keep Your Bid", "Accept Minimum Bid".
When you make a counter offer, it cannot be retracted or canceled. The seller has until 8:00 pm PST the next business day to accept, reject or counter your offer at their discretion. Once an offer is accepted, the buyer must pay the amount of any offer (plus all applicable fees) that they submitted.