250 box plants (buxus sempervirens) - what to do with them now

If you are not in a rush then I would wait till the autumn and dig up and transfer to pots, that would maximise the chance of them establishing. I'm very unsure who would want them, there are two major problems with box - blight and caterpillars, which are putting people off planting them so I expect demand would be low particularly for hedging. You might get a better chance of selling them individually as features, do something simple like pruning into globes. I would research ebay and see what is selling at the moment and at what price, you may find offering as a job lot a nursery might buy them for selling on.

 

Good luck in selling them. Perhaps a stately home or similar might want them if you can get hold of the gardener.
If they were anywhere near here then they'd be twigs by now!

 

I'd definitely wait until Autumn.

The best way to sell them would be to a nursery/garden centre but they almost certainly would ask for a Plant Passport. Technically you would need the passport to sell online as well but I've never heard of anyone being pulled up for it. To sell face to face for cash doesn't require it.