I think its sellers preference in how many he sells and to who. I think if the seller AGREES to sell you one or a few, I would expect them to honor it.
But if the same seller gets a lot of interest emails and one of those wants to buy all of them, as a seller it would make sense to me to sell them all to one person. One stop shopping.
Same thing would apply when buying.
Say I'm interested in one or two signs of a collection or one of a bunch of duplicate items:
Then another guy comes along and offers to buy all of them.
I'm going to assume the seller isnt selling anything to me and all to him. Unless he committed to selling one of them to me. Or he just calls back and says sorry they are all sold(which has happened and NOT with any oldgasser)
I have contacted sellers as well for extras they have and bought as many as I could.
(And yes Bob Richards I would have asked for a volume discount!! lol)
Once again sellers preference on what they do with their stuff.