I'm obviously not sending this without elihak7's confirmation and direction, but I think this is a believable question that can sow doubt, and can stand up against advice from his (reform?) rabbi friend (which "where's the arm band" will fail at):
"Dear seller,
The streimal you are selling looks magnificent, but I've purchased my fair share of fakes over the years, so I just wanted to confirm a few details.
Was the streimel sourced from a reputable dealer? The price you are asking suggests that this is real mink tail, but the synthetic version would go for a fraction of the price. Although you didn't post it, I assume you will include the certificate of authenticity with the product?
Sorry for questioning, but I've bought fake shtreimals in the past, and was even instrumental in shutting down two accounts that engaged in passing off fake shtreimals as real."
When he answers that he does not have the certificate, I'd offer $100, but only if shipping is free...