>How many ECGs can you sell vs how many other snake-oil products?

A lot, probably. You could sell people on the idea that it should be a part of your yearly health screening.

"And if you really care about your health you should do them every 3 months to catch problems early!"

or some such.

This will work short-term sure, but after about lets say, 3 years of no adverse findings, the average person will not take it seriously anymore.

“You’re waiting 3 months between ECGs?”