Most likely, it was some rares collector, who'd put together a collection of rares not caring whether they were dupes or not. So many true rares are server birth, or existed in tiny numbers (less than even server births, if not unique) from events and specials (some of the old pay for an in-game party, etc. events). Yet, many of these were present in the game in quantities of over 100. Every time a rares collector buys something from another shard, especially with a 7-9 digit gold value, they take a chance that its a dupe. In some cases they KNOW it is a dupe, and just don't care!
That kind of addiction to collection has led to the downfall of a number of rares collectors (think of it as the pixel version of the "cat hoarders" you see on Animal Planet, with a 1-bedroom apartment with 100+ cats). They have convinced themselves that they were not doing anything wrong, when they knew, or had reason to suspect, that the items they were buying were dupes.
I've not seen any confirmation of it yet, but I suspect the 1-account bans were a matter of EA actually being LENIENT on the rares collectors that had shopped too unwisely (and only deleted the offending account), and that in the case of actual dupers (or rares collectors that lent unique items to be duped), that EA went after as many accounts as they could find of those types.
IF EA were really being as draconian as the "victims" (and their naive supporters) claim, they would have had all their accounts banned. But that didn't happen.