Sending out snail mail seemed worth trying (we did a 5000 piece post cardmailing to a group of very likely to buy parents), but learned the hard way that we probably would need to do this repeatedly for it to pay off.
Nextdoor verifies each person's address by using one of four methods: credit- or debit-card number, landline phone number, mobile-phone number or by mailing a postcard that includes an invite code.