I've had two customers buy Antminers from me on eBay who said that they tried but could not complete a USD purchase directly from Bitmain, though I do not know the details why -- one also said he could not use his $400-off S4 coupon unless he paid with BTC.

This was a few months ago -- things may have changed on Bitmaintech.com since then...

At first I assumed people were buying overpriced mining rigs from me on eBay because they just preferred to buy from a US-based seller with whom they had some recourse if things went badly. But it seems others were paying a premium to pay with PayPal instead of BTC, either so they could reverse the payment if need be, or maybe just because they didn't have any bitcoin yet, and didn't want to link their bank accounts to the Bitcoin exchanges (one was afraid because those seem to get hacked routinely) to get some.

Extremist Libertarians, privacy nuts, survivalists, the un-banked, the government-paranoid conspiracy theorists, the money laundering drug dealers and international terrorist fund raisers.  All attracted to bitcoin like moths to the flame... :-) 

-dave (just tripping the NSA keyword-list for fun guys, lol.  Move along, now -- nothing to see here.  Go ahead... move along, I said.)