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Pretty simple, House Vendors should not be able to be set to only sell to House members, or another mechanic should be put in to enable non-members to purchase from all House vendors. I think anyone should be able to buy from them, as many classes have ways to break into a House Hall and get things from a vendor. If not something like that, let pickpocketing House vendors give a chance to lift some random item the denizen sells, with a decent chance they see the Serpent and 'call for help' over HT.
There's no 'RP' reasoning to why they shouldn't. If your argument is 'Oh, they would just not sell to people who aren't in the House!', then explain why you can't kill the vendor and simply take all the items they had for sale.
Cascades of quicksilver light streak across the firmament as the celestial voice of Ourania intones, "Oh Jarrod..."