Holmes Basic, the original 1977 Basic boxed set was the first actual TSR D&D product which I purchased, besides possibly some copies of The Dragon. I did inherit some other books before that.Though I had played for a couple of years I was very poor and couldn't afford to buy any RPG stuff. ( I did create some of my own games and even made some cardboard d8's!)
Then in 1982 I had some birthday money and went to the local gamestore to buy something.
I purchased Starstone by Northern Sages. Can't remember why. Luckily it was great and has become a classic and I now have multiple copies and have run it many times under several systems.
How about you?