If it's an aspect, they can invoke the aspect for a +2 on a resources roll.
It it's a power, for [-1] refresh, provides 2 shifts towards reducing the time needed to acquire something with a resources roll. ("I already bought it, and happen to have it right here.")
For more refresh, they can maybe do more tricks. In my game, I'd probably encourage my players to take it as an aspect at first (e.g. "Lets see what I have in my bag of tricks."), and save the refresh to possibly purchase the power later.
Also, general FATE style remedy to the "Bwahahaha I out thought you, where's your story now?" syndrome: Bribe them with FATE points to let your story happen anyway.
"Hmmm, OK Frank, I suppose you could use your bag that way, if you make the roll. For an FP, I'll even say you get X as well. On the other hand, I could give you an FP to compel your Y aspect, and we'll say that you already have Z hiding in your bag, which totally messes up your plan. I guess that would also mean the the following really cool situation develops."