I live in DC and have 18 months left on the 2-year lease I signed with the landlord.

About 2 months ago, the landlord informed me that he is selling the apartment, hopefully to another investor, who would then become my new landlord. The landlord ended up accepting an offer from an individual who wants to occupy the unit. I declined my first right to purchase as the asking price is beyond my range. I was not too concerned because I thought the lease agreement will stand (i.e. either my current landlord, or the person buying the unit would have to honor the agreement) - or I guess they could make an offer for me to move out.

To my surprise, I received a letter stating that I have 90-days to vacate the apartment because he is selling it to someone else.

I am very surprised to receive the notice and it seems oddly unfair that the landlord can demand such a thing! Is this legal in DC??

It is obvious the landlord is not aware that he has to honor the lease,
(* unleess the lease says it ends if they house sales)

So check your lease, merely inform them, that they can not evict you, since they are bound by the lease, and that the new owner will be bound by the new lease. I they are willing to buy your lease out, ( commom) you may ask for a sum of money that you would accept to move.
Common to be about 1/2 of the value of your lease, plus moving expenses. Is not out of line.

The landowner needs a wake up call that they can't move you out.
But I would perhaps have an attorney read your lease and double check and let them write the letter to the landlord

FR_Chuck's answer is not correct. Look to DC Code §42-3505.01. A housing provider who, in good faith, contracted in writing to sell the rental unit must give the tenant (1) the option to purchase the unit, essentially on terms equal to or better than the current contracted price; or (2) 90-day notice to vacate the unit. If the tenant does not vacate the unit after the expiration of the 90-day notice, then the housing provider can seek to evict the tenant through the courts.