Probably not the best time to sell a car...
Probably okay if sold locally. But how would someone travel 50-200 miles to see/buy a car be classed as essential. Even though owning a car is essential. Probably a grey area.
In the current climate (hard to find buyers due to travel restrictions and some people being financially strapped) I reckon you'll be lucky to shift an old French car with 1 month's mot for much over scrap value.
Look at it from the buyer's point of view, if mot stations are open but you're relying on the extension, there are only 2 likely explanations. 1 you don't think it will pass, 2 you don't want to spend the money. 1 is obviously an issue, 2 would say to me that you haven't spent money on maintenance. Either way I wouldn't be buying.
There are plenty of cars out there wil long MOTs at under £500 so what would you realistically expect to get for one with no MOT? £1-200? If that's the case then I'd weigh it in for scrap and save yourself the trouble of having to deal with the costs and people involved in selling it.