To wonder why people are so desperate for perfume at car boot fairs?

[crazyhairdontcare]

I was asked at least nine times on Sunday if I had any perfume for sale. And this is from the traders that invade your personal space before you've unloaded.

What on earth do they want to do with it? Is it some illegal scam I'm completely oblivious to?

Um, there are loads of used perfumes for sale on eBay, presumably in already-opened bottles. There's a whole section on vintage perfumes, some unopened but more seem to be part-used.
I only saw one listing that mentioned Royal Mail restrictions, and that
was on posting to destinations outside the UK.
I used to buy and sell perfumes on eBay myself although I haven't for a couple of years. Got some good prices too.

i wouldn't sell at car boots, people only want to pay pennies.
The RM restrictions for perfumes state no more than 150 ml (which is quite large in perfume terms) and in original retail packaging. Nothing about unused or part-used.
However, I regularly buy small perfume 'decants' and samples from well known businesses who sell said splits and decants,

They send out the goods (well packed but certainly not in original bottles/boxes) in plain vials or sprays so this regulation seems to be widely, er, disregarded.
As long as they are securely packed you should be fine.

I would be VERY happy to receive part-used bottle of Chanel No. 5 ...!