You haven’t needed to display a tax disc on your car (UK) since the beginning of October. Most cars will still have one, only those newly taxed in October won’t. You still have to pay tax of course, you just don’t get a disc any more. Police ANPR cameras have been checking your car’s tax, insurance and MOT status for years. No need for the little disc. They aren’t looking at it anyway.
If you have one on your windscreen, remove it now. Put it in the glove compartment. They are selling on eBay as collector’s items and the last issue (September) can raise £50.
I do not recommend selling a still-valid one for two reasons. First, you don’t know if it’s going to end up stuck to the windscreen of some shady character who hasn’t paid tax, in the hope the police will think their computer is wrong and not look too closely at the disc. If he’s caught, you might be an accessory. Can you prove that when you sold it, you didn’t know it was going to be used fraudulently?
Second, police computers do make mistakes. At least for the duration of the remaining time on your tax disc, you can produce it from the glove compartment if challenged.
I recommend taking it off the windscreen because if it sells for up to £50, it’s worth some thief’s time to smash a window and steal it. Thieves don’t care that it will cost you a hell of a lot more for a new window than they’ll get for the stolen item. Trawling eBay to find the vandal will be futile – there are loads of discs on there now and you can’t always see the car registration.
This time next year there will be far fewer of those discs on sale. That’s when you should sell yours.
Seriously though – people collect these things? I suppose I shouldn’t be too surprised. I have heard of people who collect fossilised dinosaur poo, so anything is possible.