In my experience I find that nationality is not a factor in good or bad experience with purchasing shells. There are some bad apples everywhere but far more gems.

Example.
I am moderately active on Internet auctions having purchased shells from shell sellers who list there, some I know beforehand, others I have become familiar with via those forums. There are private people selling shells on some of these forums who are real gems who I have had the pleasure to get to know. Some are selling shells from a deceased relative, shells they got at an estate sale, or shells they have self collected. Some shells are unavailable or next to impossible to obtain from new sources but recycled specimens from old collections is a good source. Internet auctions may actually save some shells from the landfill.
Granted there have been a few poor experiences along the way but the good experiences far out number the duds. They are not worth worrying about. There was never much money at stake. I prefer to do business in "big ticket" shells with reputable dealers who either have return policies for mail order or I can see the shell in person at a show or business.
Interestingly my worst experience on an Internet auction  involved an American seller. They sent a  poor quality substitute for the shell described then stone-walled me when I wrote to them about a possible mix up. They were a very active seller with lots of shell auctions going on at once. I finally concluded it was not worth the ulcer and moved on.  In contrast I had this happen with another American seller before this who immediately upon communication of a possible mix up refunded my entire purchase price plus shipping and said "just keep the shell." Bet everyone can guess which business person I will never buy from again and who will see me again.

There is a definite plus to face to face buying of shells. As I do not travel much and have no local specimen shell dealers to visit, I do spend far more when the opportunity to buy shells in person presents itself  than I do for the entire year combined on Internet auctions. Maybe it is good that I do not get to do this much after all!

My bottom line is take a leap of faith, get to know new suppliers of shells and for the majority of cases I find gems.

John Timmerman
Wilmington,  North Carolina