Basically, (and to repeat history you all know...) long before I was around, programmers would write their programs by hand. If they worked in private companies, they would then probably program it themselves, but if they where lucky to work at one of the larger programming institutions, then they would probably have a typing pool.
The typing pool consisted generally of ladies who where paid not a lot to just type - the advantage of the typing pool was that as you typed in (two handed) it would actually print the program on the card as well. Unfortunately, the disadvantage was that they often made mistakes in their typing.
So a skilled programmer would have one of these on their desk, and either hand punch the few cards which had been mistyped, or just punch the entire program using one of these.
I believe it works by you pressing combinations of buttons to make it punch the correct hole, then it moves the card on one slot.
I'm afraid that I don't know how much its worth - the problem is that these where actually very very common devices in the 60's and 70's, and as such, everyone threw them away - if it had been rare then more people would have saved them. I hadn't ever seen one, until I visited Bletchley park where I took this photo, and they had two! Surely they wouldn't miss one if it fell into my bag .... (joke: that would be very wrong)
The reason he want's one is that he's now fast aproaching retirment age, and he has a small collection of "training materials" through the ages of him working in computing - old 8 meg hard drives, punch tape, mag tape etc, and for several years he's been asking us to keep an eye out for one at car boots etc.
I don't know exactly how hjgh I would go to get one, but I'm not wasting anyone's time here, as I know it would mean a lot to him.