"I bought this shirt online" sounds like normal, everyday conversation.
The most likely interpretation is that you bought the shirt through an internet application like a website.
"I bought this shirt offline" doesn't sound quite as normal and everyday.
It makes sense.
It's grammatically sound.
It's something that I might use myself.
Even so, it's not what I expect to hear.
What I expect to hear is "I bought it in person."
The "in person" suggests that you physically traveled to make the purchase -- that your own body was present for the transaction.
Even before the internet existed, this sense of "in person" was in common use.
It was (and still is) used to exclude possibilities like doing something through the mail or over the telephone.
It also excludes sending someone else to do the job.
A simple Google search for "online or in person" shows about a million and a half examples of those two phrases expressing the very contrast that you want.
ell.stackexchange.com/users/12283/Gary Botnovcan
I didn't buy this shirt online.
I bought it at (name of store).
I didn't buy this shirt online.
I bought it at Muggy's.
You could also say something like
I didn't buy this shirt online.
I bought it locally at Muggy's.
or I didn't buy this shirt online.
I bought it at Muggy's on 5th Street.
or I didn't buy this shirt from Muggy's online, but from Muggy's locally (or: on 5th Street).
While the use of offline is grammatically correct, it is not the most common way to say it in everyday conversation, at least in American English.
ell.stackexchange.com/users/None/
I am not sure if it is correct, but it would not be normally used.
You could say: I didn't buy this shirt
online
To me, offline would sound like buying it on a computer, without the use of internet.
ell.stackexchange.com/users/20012/John K
Online originally meant connected to a network , usually the internet.
However, also in a company network, both clients and servers may be online .
Offline , as its opposite, meant not connected to a network .
A server could be on- or offline, or your workstation, or even a specific application on your computer.
With the raise of internet, online , in the sense of connected to the internet , became synonymous with while I was using an application that was online (connected to the internet) , as in “I bought this book online.”
It makes sense that offline , in a similar way, comes to mean: A)
while I was using an application that was offline (not connected to the internet).
However, it seems that offline more and more gets the sense of the opposite of the new meaning of online , rather than evolve from the old meaning of offline : B) while I was not (using an application that was online (connected to the internet)).
These two meanings are quite different.
I assume that most people would read your sentence as having meaning B, but you do risk people reading it as A, in which case your sentence makes little sense.
ell.stackexchange.com/users/2992/oerkelens
Offline could be used in a situation like that, but is generally not the most natural/common way to say it.
I might say that as I bought this shirt at [name of store].
Or, if the context makes the meaning clear (e.g. someone asked Did you buy that shirt online? ), simply...
I bought this shirt in a store.
(being American, I'd use store here
, I believe British-English would use shop )
If you're conversing over an informal text-based medium (e.g. text messages, IM, etc.)
you might use the acronym IRL ( in real life ).
I bought the shirt IRL.
ell.stackexchange.com/users/3329/Tim S.
As in any language, words and usage evolve in English.
The word offline is commonly used to refer to retail stores (do a Google search for "buy offline").
Though offline is not the most common way to express the idea, it does it very well and is going to become a part of my personal usage.
So @Bharath Manjesh, I would say your use of offline is on the leading edge of English language development.
ell.stackexchange.com/users/11495/Ast Pace
While offline may be coming to mean "not using the Internet", it cannot be used in this sentence.
There is a common ( millions of Google hits documented )
phrase I bought (or ordered or got) this item off (name of source).
and someone hearing you say
I bought this shirt offline.
is liable to misinterpret it as
I bought this shirt off Line. and ask you why they've never heard of this store before.
Your usage has also been discussed on the English Language Usage and Grammar site where the clarification of "I bought it off the internet" is suggested, which is the exact opposite of what you mean.
Don't do this.
ell.stackexchange.com/users/5039/Ben Voigt
"I bought this shirt  offline ." Is 'offline' okay to use to refer to something that was bought at a brick-and-mortar store? If not, what's the preferred way to say it in  everyday conversations ?
"I bought this shirt offline." Is this correct usage of the word 'offline'?
ell.stackexchange.com
<word-usage><word-request><usage>
ell.stackexchange.com/questions/57788
ell.stackexchange.com/users/None/
One might say "I bought it in person", "in real life", or "in a store". Alternatively, "I didn't buy this shirt online." While the use of  offline  is grammatically correct, it would not be used in this context. This is not commonly used even if it is grammatically correct. You could say 'I bought this shirt online/IRL/at [name of store]/in person.
You could say 'I bought this shirt online/IRL/at [name of store]/in person. This may be grammatically correct but it is not used.