The DVLA is still dealing with delays due to coronavirus. I would never advise a buyer to purchase a car without a V5. You're essentially shifting the issue you're currently facing onto them, so it's not exactly a good idea. Is it legal? Yes. If someone buys it, they can send off a V62 form to get the V5 but - again - this will also be delayed due to the DVLA's issues with processing paper applications so then they won't be able to tax it until that's sorted.