You are letting your common sense and logic getting in your way. Throw. That. All. Out.
Just let it ride.
(This is a perfect Axe Cop
This 29 year old guy drew whatever his 5 year old kid brother made up and barely changed anything... And it turned into a multi-volume comic series with a minor cult following.
Reasons and rationals are useless in these situations.
(Actually with how people are like nowadays... Those ain't that useful in daily life either...
Relax. Enjoy yourselves first. Everything comes after.
Well, something a little more practical:
When something like that happens again, instead of thinking why wouldn't it work, think:"What is gonna happen if you say yes?"
(A variation of the "yes, but---" rule some freeform games used.)
And go with the more entertaining results.
(This is pretty much how I run all my games.
Which included things like everybody jumped off a building one after another and only one PC got hurted, or that for some reason every place a team went to always got blown up unintentionally... Fun times.
in this case:
So now, next:
>"Where can you buy it?", then
>"How can you afford it?", then
>"Where can you get the objects to trade for it?", then
>"What do you need to prepare for getting there?", then
>"What are you going to face when you get there?", etc.
Adjust with your kid's intention every step of the way, and/or change to something else if/when needed.
And if you have problems just making up stuffs on the spot, take a page from the old movie "The Usual Suspects."
Seriously every GMs who do freeform should watch that at least once.