I don't think the GLWS is a problem as it can keep a for sale thread at the top of the section. This in turn keeps it in people's minds.
Arguing over price is wrong. I've nothing against someone saying "Mate your price might be a bit high as they are for sale at X,Y,Z for £X, £Y, £Z in a similar condition to yours" This could help with the sale but they should leave it at that.
You will get some people that will try and take the piss and Not everyone does research and you will sometimes get people trying to get back what they paid for something 18 months ago (i'm talking parts/gadgets etc here not cars)
But most will price stuff fairly. So others shouldn't start a bunfight over price. Locking the thread is probably the only way to prevent this.