Are supermarket blood pressure machines reliable? Are the blood pressure machines in supermarkets generally reliable?

Possibly. Sometimes they are but if not maintained, or if you are not using carefully, do not trust them. Always get BP checked by a reputable provider.
Supermarket . Supermarket blood pressure machines are prone to several sources of error, but the inaccuracy is not dramatic and they are useful in many circumstances. If you obtain a high BP (>140/80) on one of these machines and are not already diagnosed with hypertension, then you should see your doctor to confirm the measurement and discuss possible treatment. If you are already diagnosed with high BP and are being treated or observed closely by a physician, then using such a machine to trend your BP and keeping a written log of your bps is useful. This information can help your doctor decide whether to start or adjust therapy.