Honestly, Dabi's had some wonderings about what would happen if a vampire showed up. Considering blood is both a source of power here and a source of corruption, someone who relies on it for sustenance would be in an interesting dilemma. He knows a person who uses blood to run their own power, but so far they haven't turned into something disgusting...

"Yeah, but a lot of them don't come back though." Or if they do, it's not as themselves. And there went any spoils of their search with them. Unless another fungus fanatic comes along and makes off with the ill-gotten boon. Mayerling's able to gather them without issue, so therein he has a good curb on the market. Might as well tip him off to another potentially interested party.

"There's another stall further down that sells incense. I'm restockin." Just because he's not afraid of Beasts and can hold his own well against them doesn't mean he wants to invite them into his place on the random chance they come by. Good to have some warding abilities on hand if necessary.