Which is correct: "I bought it online/offline"

On several occasions, someone has asked me where I purchased my shirt and I replied with "I bought it offline."

It didn't sound right to me.

My brain thought that I bought it 'off' of something (as in "I bought it off craigslist"). But the noun 'online' confused me (I couldn't remember the name of the website). "I bought it off online" just doesn't work, and"I bought it offline" sounds like you didn't purchase it on the internet.

However, "I bought it online" doesn't quite satisfy me either, I am not sure why :P.

Is one more correct than the other? Is there an all around better sentence?

Only the phrase "I bought it online" is correct.

Saying that you bought if offline indeed makes no sense (unless, perhaps, you intended to make some sort of joke). "I bought it off online" doesn't make sense so avoid that one.

If you don't like the phrase "I bought it online" you could say "I bought it on the internet" or "I bought it from an online store". "I bought it off the internet" is also an option if you prefer to use the word "off".

I bought it online is correct, with online being used as an adverb. I bought it offline would mean that you bought it while not connected to the internet which is clearly not the case.

"I bought it online" is generally the phrase I would use, or "I bought it while online," or "I bought it from an online store," to distinguish it from a bricks-and-mortar store (which is what "I bought it offline" would suggest to me).