...about sales, and the guy trying to sell his book talks about how to "overcome any objection".

Geez, if someone simply doesn't want what you're selling, you can "overcome any objection">?>? I've had people come to my door with soap or encyclopedias (OK, that was years ago) and had to shake my head in a sadness at how hard they tried.

Do they teach this at MTC (besides a tearful testimony)? Hard-sell things like "Do you really want to jeopardize your eternal family?"

There is no salesman ship taught to the missionaries. If they every approach you, they come across like they are making it up on the fly which they are.

Isn't there?
Back when I went ('79), we got quite a bit of instruction in salesmanship.
They just didn't call it that.
They called it "how to get people interested when they aren't interested."

But it was all about overcoming objections.

Yup sure is. It is called 'the commitment pattern' google it and get a bunch of sales/business hits. The MTC just dresses it up in glitter and prayer.

I don't know if they still use that but they did when I served.

Eternal family = straw man