Many of the great ancient coin collections are in Europe. Many of the great ancient coins auctions are in Europe. But buying coins at auction in Europe now presents an entire array of obstacles and difficulties for the American Collector.
First, most American collectors now prefer to buy graded/authenticated coins. You won't find these at European auctions. Each auction house has its own style of photography and coin grading. Even if you are very familiar with the style of a particular auction house you'll find that coins of dubious authenticity, and coins that have been expertly altered, smoothed, tooled, repaired will be included in many auctions without description.
Furthermore, if you send these coins in to NGC after receipt and discover a problem, then go back to the auction house, you will be told simply that they disagree with NGC. End of story. These auction houses have their own experts who defer to nobody.
Of course, if you are a major client of an auction house, and develop a relationship with them over time it is certainly possible to better your odds of acquiring a problem free coin. But even then, their views of what constitutes a problem free coin may still be different from the views of NGC.
But, let's just say you have a terrific eye, or trust a dealer with a terrific eye and you buy at a European auction, and manage to avoid these obstacles. You are now faced with the problem of importing the coins from Europe to the US. You can either trust Registered Mail, which can take up to several weeks, can not be tracked, and can take up to several months or even years to recover insurance payments for lost shipments. Or you can trust a carrier like Fedex or Amit Malca. The problem here is that Shipments into the US containing ancient coins are now routinely stopped by Customs. Once your shipment is in customs there is no time table for release. It is entirely up to the discretion of the customs officer. If a coin is seized there is no recourse or second opinion. Your coin is gone.
Once a coin is in customs. Fedex washes its hands of the entire process. You'll get no help there. Amit Malca will try to deal with customs for you. For this they charge thousands of dollars, so you can add that to the cost of the coin.
Now, most shipments of legal coins - not on cultural property lists, if properly documented, will most often be released in a timely fashion, if seized. The problem is that establishing provenance for a coin is not always a straight forward process. And cultural property lists are expanding.
It should be said that there are always ways to navigate difficulties and narrow the odds that there will be problems. Especially if you have close personal ties to a European auction house. But the entire importation process is becoming more difficult by the day. The gap between the European and US markets is only widening.