Thanks everyone!
That is reassuring, anyway. I think our freezer is at least fairly good, and they have only been in there around 6 months at the most.
I don't, so thats not possible unfortunately.
The problem here, (which I completely forgot to mention, sorry!) is that my flock is MD positive.
I guess I could say in the add, 'they are from a Mareks positive flock, so are carriers. Don't buy if you have your own chickens!' But it still don't seem a very good idea, and most likely everyone would be scared that they would get it as well.
I think the thing to do, is just try and pack them extra well, both to keep, and also so they thaw out quicker.
And maybe get another freezer, which is kind of in the pipeline anyway
Maybe I just need to learn to cook myself...?