Just wandering...

Why wouldn't it be?

If you can put a sign on your house why not your car.

Just do it and worry if anybody complains!

Sometimes I "scratch my head at posts on this site". There is nothing wrong with placing your vehicle for sale, also putting a for sale sign within the said same item. However make sure the vehicle is parked in a non obstructive way, ITV in place, Road Tax and Insurance if parked on a public road. I hope you sell the vehicle. Out of interest, what make, color,  2 or 4 door? year, ITV left. Petrol or diesel?, manual/automatic? How much are you hoping to sell the vehicle for? As you have not sold/transferred  the Vehicle December 2023 end you are also liable for this years road Tax 2024. I hope this helps you. Sincerely. R. 

Its good to advertise your car here it is free.

One can understand the confusion when Murcia Today reports that putting a se vende sign on your car, whether being driven or parked up, is illegal and can lead to a fine under the law of "surreptitious advertising". 

You can read about it here Se Vende

A chat site in Andalusia reports someone put a se vende sign on their car parked up outside and came back to see a fine stuck on it for 300 euro for showing an "illegal" advertising sign on his car.

Several other papers have reported the same thing.

On the other hand, I see cars all over the place with se vende signs and a phone number on them and no sign of a warning.

I know there are some funny laws in Spain (like there in UK) but it can be a bit tricky to know which laws matter and are still in place. Hence, I assume, the OPs question.

(Tried to look at Citizens Advice Spain but that article is password protected).

I have always valued your input marcliff. You are one of the better people on this site. No vulgarity in any of your postings unlike some others.  Your posts 99.9`% of the time make sense. I live in the area that comes under Alicante. In 15 years I have seen signs (For sale/Vende) on all sorts of items, the Guardia don't bat an eye lid. However I suppose that does not mean one is not breaking the law in another region  then. I personally have never heard of anyone being fined, people in my area have also put signs up for land for sale, spray painted property for sale on the outside of a residential wall for all to see. This is approx 300 meters from our Town Hall. To reiterate no one has "battered an eye lid". I have travelled all over Spain, observed "Vende" signs in many Regions, followed vehicles up and down the motorway with Vende signs obscuring the vision of the driver. The Guardia have driven passed the vehicle, nothing happened. The world is a wonderful place to live, slowly ruined by mankind with ridiculous rules and regulations. Kind Regards. R

I think you missed the point of my post. Just pointing out how confusion can set in with articles in papers and I did say I've seen loads advertising. 

Hello. I am 91, Confusion is also something that comes naturally with old age. My brain does not function as good as she used to. Keep up the good work on this site.  My apologies. R.

You have to ask your town hall.

It's strictly forbidden throughout Spain whilst the car is being driven.

Whether or not it's allowed when parked depends on where it's parked (it could be considered misuse of public road) and local town hall rules. Usually it's not allowed. Whether or not anything would be done about it is anyone's guess. You might be lucky, you might be unlucky.

It's illegal to do so in London .