We're playing Felltower this weekend, and naturally this means planning what to do.
Cue up a long list of places to go, most of which have an attached caveat about equipment needed to pull it off.
Pretty much, every player plan seems to depend on everything going right, and on some very specific equipment requests.
These requests are often qouted in a Princess Bride reference, but this is actually what goes through my head as people plan equipment:
It's never "a holocaust cloak and a cart." It's "8 charged scrolls of (obscure spell), two spellstones of (specific spell) per person, special order equipment, and one (rare item not actually for sale.)" It's always some multitude of things that would have taken a lot of planning. It's a plethora of pinatas. It's 10,000 marbles.
Generally I've dealt with this by saying you can't shop for availability, you can just shop. You try to round up what you need from a limited supply spread across multiple merchants. You can't say, "Let's see if they have 10,000 marbles. If so, we can decide if we want to do this. If not, we don't buy the marbles . . . this time. Next time, we'll know they have 10,000 marbles." What a nightmare.
It's as bad as that time my friend figured out that with some time manipulation and scrying magic in Mage: The Ascension we could reasonably fight out a fight precognitively and then decide if we won we'd actually go through with it. Funny, maybe a great movie or book idea, but in game terms it's useless grinding of time that could be fun and making of misery for the GM. The GM spends a lot of time coming up with answers that may or may not be relevant.
So basically, if you want to know they have 10,000 marbles, take your money out and shop. If they do and you don't get them, they're no more or less likely to have them next time. Do it too often, and it goes to "less likely." It's a GM and player time saver more than anything else.