Perhaps you are buying a

Doughnut (though not!)

EDIT: After being told the first one is not a correct answer, here is another one:

It is COTTON that you are buying, as

I started off buying nothing.

Nothing = O

Then I bought twice as much nothing.

Twice of nothing is OO

Then half as much nothing with a whole lot of nothing.

half as mucj nothing is C (half of O) with whole lot of nothing is NOT

At the end, only half of what I was buying was nothing.

in the word COTTON, half of its NOT, as per the title of th riddle (Not can be used for nothing)

Perhaps you are buying a

Doughnut (though not!)

EDIT: After being told the first one is not a correct answer, here is another one:

It is COTTON that you are buying, as

I started off buying nothing.

Nothing = O

Then I bought twice as much nothing.

Twice of nothing is OO

Then half as much nothing with a whole lot of nothing.

half as much nothing is C (half of O) with whole lot of nothing is NOT

At the end, only half of what I was buying was nothing.

in the word COTTON, half of its NOT, as per the title of the riddle (Not can be used for nothing)

Perhaps you are buying a

Doughnut (though not!)

Perhaps you are buying a

Doughnut (though not!)

EDIT: After being told the first one is not a correct answer, here is another one:

It is COTTON that you are buying, as

I started off buying nothing.

Nothing = O

Then I bought twice as much nothing.

Twice of nothing is OO

Then half as much nothing with a whole lot of nothing.

half as mucj nothing is C (half of O) with whole lot of nothing is NOT

At the end, only half of what I was buying was nothing.

in the word COTTON, half of its NOT, as per the title of th riddle (Not can be used for nothing)