Yes, I've been to gun and knife shows. I can see why sellers who specialize in Nazi insignia would want cash. At gun shows here, it was always the guy selling reloads without the appropriate license or insurance that wanted cash. Like I said, it's the first red flag. All the rather thinly veiled white supremacist stuff is why I don't go to gun shows.
Personally, I've lost interest in things I can't buy or own above board, legitimately.
That happened right around the time I became able to buy (and afford) just about anything I want legitimately. An above board piece of merchandise is almost always better than it's illegal counterpart, in no small part because more money is involved, and reputations are actually on the line. The more skin everyone has in the game, the higher the profit will be and, therefore, the better the product will be. You think you can make a lot of money under the table? Take the same business legitimate, and make a fortune.
Cash, on the other hand, severs the last tie between buyer and seller the moment it's handed over. That's why drug dealers use it. (No, I'm not saying everyone who uses cash is a drug dealer, I'm saying this attribute of cash is well known.)
There's nothing more worthless than a weapon you can't use because you're not supposed to own it. A weapon that you're entitled to, on the other hand, that you bought legally and have every moral and legal right to have in your hand and use if you must (and because of that situation is of the highest quality a capitalist society can produce) can be a serious problem for bad guys.
Brownshoe, a discount that large should tell you something.