Sforno on Genesis 25:33/ספורנו על בראשית כ״ה:ל״ג

1השבעה לי, seeing that the purchase under discussion was something abstract, the oath would replace the act of physically taking possession of the object purchased and moving it.

2וימכור את בכורתו, in accordance with the price they had mutually agreed upon. The Torah did not bother to spell out the exact sum.