I concur with Mr. A’s answer and I suppose that in some Tagalog regions, “magsadya” is still rather commonly used. However, in metropolitan Manila nowadays, “magsadya/sumadya/sadyain*” is only used in formal conversations and usually by the older generation. “Magpunta/Pumunta/Puntahan” has practically replaced it and the nuance of “intent” would just have to be assumed when applicable. Aside from its meaning of “going with a definite intention”, it can also mean, “going out of one’s way to go someplace”. Usually, you will hear it used when purpose is being explained by one paying a visit with a person of authority. To use it in casual conversation might instead produce a melodramatic or comic or even a sarcastic effect. Thus, "Magsasadya na ako sa palengke para bumili ng gatas" (I am now purposely going to the market to buy milk), although grammatically correct, would come across as histrionic and/or comic. When said to a particular person, it may imply sarcasm as it can be understood to mean, “I am now purposely going (and going out of my way) to the market to buy milk (because you demand that I buy it specifically from there when I can just conveniently buy it from any store nearby)”. “Magpupunta/Pupunta na ako sa palengke para bumili ng gatas” would be the straightforward and best way to say it. * Please note that “sadyain” is also the word used to mean “to do something with malicious intent or with a self-serving purpose”.

As you have stated, magsadya is used like 'to go to a place with definite intention. ' For example: 1.Para sa iyong katanungan, magsadya lamang sa aking opisina. 2. Nagsadya ako dito para lang kunin ang aklat na ito. Magsadya is not commonly used these days in spoken Tagalog. But in my town in Cavite and Batangas it is still used almost equal to pumunta (with intention). In your sentence: " Magsasadya na ako sa palengke para bumili ng gatas." It is correct but you can also say: -Magsasadya ako sa palengke para bumili lamang ng gatas. Notice that I used "lamang" or "just" to indicate that I intentionally went to the market just to buy milk. I don't have any reference/book in my answer, it is how we use in our town. Sana nakatulong ito sa iyo,

Magsadya is the opposite of naligaw lang. Naligaw lang is just to have accidentally passed by that place and not really willfully or purposefully. Magsadya is to purposefully, willfully, intentionally and with all your heart go to somewhere or do something. Sadya is to willfully and intentionally and with all your heart do something. And not as if someone was just forced to or compelled to or unintentionally do something.

Uhm, I have never heard that kind of verb too even though i am a FIlipino and a Native speaker of that language. Mostly, we only use "Sadya" as in "To do it on purpose" For example, A guy suddenly bumped into you on purpose and then he says "Sorry, di ko sinasadya" "Sorry, i didn't do it on purpose" or "Sorry, it wasn't my intention" Then you say, "Sinadya mo kaya!" "You did on purpose!" in a casual way. I never heard anyone say Sadya in another meaning. Did you misheard it or... Maybe i just don't know the wordXD lol. Pero sa tinging ko mas okay nalang sabihin mong "Pumunta" kesa/kaysa sa sabihin mong "Sadya" kasi appropriate naman eh :) Di naman ata necessary yung sabihin na Sadya hahaXD