In the used market its all or nothing. You pay the price or you walk away.
In most markets though there is the opportunity to offer.
When I am selling a plane, maybe the market is slack and I might like to receive an offer less than my asking price to move the plane on.
When I am buying, maybe I see lots of one model or one plane has sat around for a long time so I might buy but not at the asking price.
Perhaps you could when listing a plane also set an 'auto accept' limit - so plane is asking $10,000,000 but seller will auto accept $9,700,000 but anything less is sent to the seller for consideration.
This means a lower bid they buy has to decide if they want to risk losing the plane or pay the asking price, because maybe someone else comes along