Require retail gun buyers to provide a seller with 2 or 3 attestations that the buyer is someone they've known well enough (for at least some minimum period of time) to honestly attest that the buyer does not have the personalty traits of an antisocial disturbed individual.
Most disturbed loners wouldn't know individuals willing to sign such statements with their names, addresses, and contact phone numbers.
No one is being asked to judge if the applicant is mentally disturbed. Advantages are:
1) Be more effective for preventing weapons sales to some mentally disturbed individuals;
2) Involve less government bureaucracy and cost to implement;
3) Pose no challenge to personal privacy;
4) No new reporting burden on schools or therapists; not discourage patient's openness in therapy.