Old Silver Bar Sold On Ebay

The lack of provenance is scary. The lack of seawater effect means this was almost certainly not a shipwreck recovery.
88g is also extremely light for silver bars in this form with most weighing well over 0.5kg to 1kg or much more.

The facing side shows three tax stamps, or assayer stamps, with partial stamps visible on the sides, meaning this was probably "plata corriente" or cut-down pieces of larger bars which were intended for coining. These stamps are much more ornate than the rather crude stampings seen on Spanish bars of the same era or the tapered VOC "bonk" bars with each end stamped. The fact that the date "1654" is written in Arabic numerals on the two right stamps is also a concern, since almost all silver bars of the same era used Roman numerals exclusively, as in the first stamp.

The leftmost stamp seems to show MDCL (i.e. 1650) at the top but may be cut down from MDCLIV or 1654.

When compared to shipwreck recovery bars from the Capitan & Atocha, as well as non-recovered silver from Germany and England, this one looks far more modern.
There are no fineness stamps that I can see, no assay punches, and no owner marks, the weight and size are not in line with the averages, and there are no tally marks visible.

My inexpert opinion is that this is a modern fantasy bar or slice and not an actual piece of mid 17th c. silver. If you can somehow match the stamps with known and authenticated bars, that would probably help; you could also take it to a museum or university which has a curator or professor who is well versed in mid 17th c. gold and silver ingots, bars, and slices. I would not buy this without an assay report (preferably XRF to look for expected amounts of tin, copper, lead, and occasionally arsenic) and an ironclad return guarantee, and this seller does not offer returns at all.

Edit: This sold for $504.00. 88 grams of silver at .925 is valued currently at around $45. Silver bars of this era usually had fineness around .885 to .920. I'm assuming that he meant "grams" and not "grains", because 88 grains of silver is a measly 5.7 grams.

Not directly related to the authenticity of the bar, but this seller has three negatives which accuse the seller of shill bidding (and name the ID).

The bidder named has 627 feedback.
This silver bar has a bidder with a scrambled name containing the letters of the ID named and a feedback score of (627).

To me, this means they shill bid on this silver bar as well.

Note: the winning bidder is not the same bidder that bid $99, and the winning bid came in after the supposed shill bidder's bids.

The seller's location is Funcal, Portugal. Funcal is a city on a Portuguese island named Maderia.

The supposed shill bidder is registered as being from Maderia, Portugal.

Fake stamped bars/ingots seems to be a fun game out of the Iberian peninsula the last few years...