Its not a good disease but it can be managed with good practices, exercise,feeding[nutrition] if the horse is a carrier on one side. If the horse you sold has papers depending on age at the very bottom under the owners name should be 1-2 sentences that will say this horse is known to have an ancestor which traces back to Impressive, then it will either say it is recommended that he/she is tested or that they are tested and the results>N/N that they don't have nor carry the gene N/H that they are non systematic but do carry the gene H/H that they are both systematic and carry the gene.
Regardless of the results it is in everyones best interest not to breed horses that have that lineage[in my opinion] There are enough good quality horses out there that there really is no reason to breed to what is know to have a genetic defect.
Now as far as the sale, that is a moral issue, If you sold the horse as a breeding animal the buyer being a responscible breeder should have been aware of the HYPP factor.
If sold as a riding animal, if it is H/H it can have seizures and really is unsafe to ride, in good faith you should take it back, although an honest mistake, better safe than sorry and I'm sure you would not want to be responscible for someones injury.