Market Acceptable - Where Did This Term Come From And When"

I think it just means that a typical buyer, or even a TPG, would not penalize or qualify a coin for minor issues or perhaps an old cleaning that has toned back nicely.

There are two or more grading definitions that apply to specific coin types.

First is called a technical grade. The only determination of the grade using this stricter judgement is based on a very specific degree of rub and wear for circulated coins. Mint state is judged by the number and placement of dings, scratches, bag marks, etc.

Notice that neither of those are affected by degree and brightness of luster, strike (even those types that do use strike as a factor, such as full head on Standing Liberty quarter or Franklin halves with FBL) do not change the numerical grade. Nor does toning or lack thereof, or proof like or any other secondary consideration.

The number for the grade is very consistent because none of those other conditions change that number. Eye appeal and desirability most certainly do affect the value of the coin. But those other effects do not change the numerical grade.

Damage will kick a Coin out into details, just like any other current TPG grading.

ANACS made the unfortunate decision decades ago to give coins a mixed grade. A coin might have been a 35 with technical grading, but because it had a prominent scratch it was given a net grade of 20. That decision of how the net grading worked was virtually impossible to understand.

There are still people who use this net grade definition.

Market grade takes the "look" of the coin, to include strike and luster and toning into consideration in the final grade.. in some instances older pre-1810 coins have been chemically cleaned but show no parallel lines from mechanical (hand) cleaned coins won't kick that coin into details. It will be said that the final grade is market acceptable grading.