Difference between intention to convert and intention to own

Hello Rabbi,
 
I wanted to ask about the comparison between intention in conversion and intention in purchase. The Rabbi argues that just as a person buys only if he has the intention to buy, and the laws of purchase are only the manner of execution, so too in conversion. There needs to be a basis, a precondition, of intention to accept the mitzvot, and the laws of conversion are the manner of execution of the conversion process for someone who wants to accept the mitzvot as a prerequisite.
It is clear to me that in order to buy, you need an intention and a desire to buy. Here I will note that the rabbi said in a lesson about the Seder and its commandments that someone who did not intend to buy but performed a proprietary action, did not buy. I do not understand this. How can one turn to halacha to make a purchase according to halacha, without wanting to make a purchase according to halacha?
But in conversion, one can enter the process (turn to the law to carry out the acts of conversion) in order to join the people for other reasons, of course, and not out of a desire to maintain a religious lifestyle committed to the law.
I will try to clarify, even for myself. In a purchase, the one who inquires is a religious person who has opened a Shulchan Aruch (or inquired in some other way) and wants to make the purchase according to the halakhah. There is an internal halakhic desire prior to clarifying the halakhah. A convert who does not want to observe a commandment does not have this “halakhic desire.” He turns to clarifying the external halakhic acts that must be performed in order to be recognized as a foreigner, as a Jew, but without the internal halakhic intention. Why is the situation of these two people the same? Why attribute halakhic desire to both? Both people have a motivation to perform the halakhic procedure, but only the buyer has a halakhic desire. A convert of this type has an extraneous motive for performing the halakhic procedure, and not a desire to receive a commandment.
I hope I made it clear…