1739 8 Reales Better Pics (Authenticity?)

Honestly? You should use all the obvious scratching/tooling marks (which weren't mentioned nor discernible in the auction photo) as your legitimate excuse to send this dreck back from whence it came.

You're better off saving up the $150-175 for a decent wreck piece which may have some porosity or even a spot of corrosion by the rim, but has XF-AU detail and is presentable to look at.

From my perspective the coin is too worn to be positive from photos without much closer shots.

I would examine this one at probably 100X to examine protected transitions between features and the field.

Weight should be specified to one hundredth of a grams

Specific gravity would be better expressed to two decimal places (accurately).

Regarding the Mo - I am not convinced that was a single punch consisting of a tiny o over the M. It was clearly a single punch for later years but 1739 is very early to presume one punch.

Both of the M's and basically every other letter and numeral on the coin is damaged. A better level of magnification might be able to weed out damage and what it did not the characters.

When I saw the coin earlier I commented that the damage made authentication in this case VERY difficult if not impossible from these photos.

The edge looks bad - I presume damage to the edge is the reason but in pictures there is no way to assess if what appears to be there is actually there or due to various hits.

Finally the dark corrosion or whatever on the surfaces add to the difficulty.