It's not just valuable stuff that gets stolen at autojumbles. I used to share a stall with my brothers at the Spring and September events.
When I had boxes of taps and dies for sale (duplicates of a very large lot I had bought previously) I reckon about 50% of these were taken without payment. These weren't valuable items; I had already binned any that were damaged and IIRC, was charging £1 each for HSS taps and 50P for the carbon ones.
This, and other thefts from our stall left a really bad taste for me, and is one of the reasons I now attend these events as a buyer and not a seller.