Most "guild" vendors I have seen are simply alts created with vendor-ish names and put into merchant mode. you can have multiple people selling from the same room. There are limits to this but I don't know off hand what they are and do not care enough to go find out for you when you can do that on your own time
But I believe it is increased based on house size same as house item limit is.
Keep some things in mind. First of all, the vault only works for the toon who purchased the house. If You purchase the house with your Main, Sirmaimsalot, and you want to create an alt on the same account named, lets say, Sirsellsalot, you can place Sirsellsalot in Sirmaimsalot's room, but Sirsellsalot has no vault access. also, as soon as you put Sirsellsalot in merchant mode then any items that Sirmaimsalot has in the vault and listed are no longer listed for sale. Only one toon per account can be selling in game at any given time.
You can have any guildees or friends or whomever come to your room and park there in merchant mode as well. They simply have to access the bulletin board, list their items and voila!
Hope this clears the mystery up a little bit, Velma.
Paddyo Fer'Niture
Level 40 Templar
the blessed House Al Nasir
Antonia Bayle Server