When the missus took her Ka to trade it in as part exchange against a new Aygo Blue the bloke at the Toyota place offered £4300 in part exchange. We agreed, but when I got home I went to the (in)famous "Webuyanycardotcom" sharks, who offered £4700. We had to take it to a local garage which was an agent for said sharks, who then proceeded to find "problems" (there were none which we hadn't told said sharks about, being mostly stone-chips which had been properly touched-in and were only visible on very close inspection) and downsized the offer to £3200.
We told them to stuff it, and I eventually put it on Ebay and sold it immediately for £4600.
The moral in there is easy.