Tiberius? ID Please

Is it possibly Greek? Seems to match some Greek coins with an eagle.

So you think it's possibly a Tyrian shekel?

Unlike most other Middle Eastern countries, tourists are allowed to buy genuine ancient coins in Israel. So there's some good news: we don't have to automatically assume that it's fake (which is something we would have to assume if you'd said "in Ephesus" or "in Cairo").

That being said, I don't hold out too much hope for the knowledgeability of the "reputable antiques dealer". Because, assuming this coin is genuine, it is not a coin of Tiberius; it is not even Roman. This is a tetradrachm of the Egyptian king Ptolemy III, who ruled long before the Romans arrived on the scene in the Middle East; it is dated to the 8th year of his reign, or 239 BC.

Given that the seller was so thoroughly wrong with the ID, I would not trust them to be able to tell the difference between fake coins and genuine coins.

There is a good chance this is a genuine coin of Ptolemy. Here is some information that might me of interest to you. http://www.houseofptolemy.org/housenum.htm

Could it have been USED at the time of Tiberius? Maybe there was a misunderstanding.

I can't say whether to keep it or not as I am mainly concerned with Macedonia and the earlier Successors e.g. Seleucus I and Ptolemy I. In terms of worth, 300USD seems in line with retail prices in the market. On a bit of research Ptolemy III bronzes seem far more common than silvers so you have an upside.

In terms of being genuine the style is consistent with other pieces of the time and the toning is consistent with how they are found so I would say it is genuine from the photos provided tho check if it has a line around the rim of the coin as that will indicate faking through the casting process btw I am Aussie too :P

It is a tetradrachm of Egypt, not a shekel of Tyre, but it looks very similar to the shekels of Tyre and the two weight standards are close enough that, if a coin of Ptolemy III had been found during the time of Tiberius and put back into circulation, I don't think anyone would have paid much attention. But it would have been an extremely unusual occurrence. A silver coin simply doesn't last for 200 years in continuous circulation; 100 years is pretty much the upper limit in ancient times. I suppose it could have been lost and then dug up again a couple hundred years later, but you have to remember that prior to the invention of the metal detector, lost coins tended to stay lost. So connecting this coin with Tiberius is, basically, not logical.

I suppose it's possible that there was some sort of confusion. The dealer might have said that it was "found near Tiberias" (spelled with an A not a U), which is a city on the shore of Lake Galilee; the city itself did not exist during the reign of Ptolemy III but the Galilee area was under the control of Egypt at the time the coin was issued.

Finally, an important distinction should be made: in Israel, there are "reputable" dealers, and there are "licenced" dealers. Only "licenced" dealers are legally allowed to sell genuine ancient artifacts (including coins), and there are only 65 of them in the whole country. If the dealer doesn't have a licence, then the coin is either fake or looted/stolen. If the coin is genuine but the dealer is unlicenced, then the coin cannot be exported and getting caught trying to leave the country with it could in theory get your girlfriend fined $30,000.

Edit: Ben typed while I was typing all this.