I haven't bought the shed in several updates, so I'm not sure how similar it still is now, but you should get a message pop-up that interrupts your dialogue, asking you if you want to "buy" the shed key for however much you haggled for it.
You should specifically mention you want to buy the shed, and you'll probably see it pop-up after Gareth replies. (So read fast) Just say no and start haggling the price down. Though, you may get the pop-up after every price offer from Gareth.