
£150 offer countered by £1000.
So we've now established he'll definitely take two thirds of what he's asking.

This is reciprocal p*ss-taking.
Asking £1500 is clearly a gambit. The result of this is his counter offer, which is an already enormous lowering of his trousers. Odds on if I wanted the car it'd wind up around £500, probably without the head unit.
For those struggling with this whole process, it's called 'haggling'.