Why "B>TUNA"?

probably hoping they catch that one noob who knows nothing about tuna

Basically this.

Merchanters know that Tunas are very rare--almost nonexistent now'a'days. And despite that, they're still fishing for that one unknowing player, that was lucky enough to have got it, to sell it to them for a low, low price.

Same thing with those merchanters who spam about buying certain rare chairs or Hwabis. They're all hard to find, but they still try to catch someone on their lure.