Not sure about the legality of reselling it, but I"m sure you could find people in the area who would be interested in sharing it and thus making it more cost-effective for you. I for one could be interested in future batches (I don't have any chicks just at the moment). Just asking around and finding people by word of mouth would be fine.
Sharing it and not making a profit (but covering costs including freight) couldn't pose any legal problems.
I would be a bit hesitant to actually advertise it, because you would then be opening yourself up to potential problems with the supplier, and there would be questions of liability if someone bought it and used it and it didn't work - ie. their prized rare bird dropped dead of Marek's when they thought it was vaccinated. Mightn't be your fault, but it could get ugly.
How long does the freeze-dried, undiluted vaccine last in a normal fridge?
How good are you at giving needles?? That's the only part that worries me about doing it myself!